From 44a4517e2b1189bde950b86cbf6c2c5943a5a55a Mon Sep 17 00:00:00 2001
From: Charless Milette <charles.milette@gmail.com>
Date: Sat, 13 Nov 2021 11:29:05 -0500
Subject: [PATCH 01/21] Support C++20 std::format as an alternative to fmtlib

---
 CMakeLists.txt                                |  15 ++-
 appveyor.yml                                  |  19 +++-
 bench/async_bench.cpp                         |   4 +-
 bench/bench.cpp                               |  14 ++-
 include/spdlog/common-inl.h                   |   4 +
 include/spdlog/common.h                       |  48 ++++++--
 include/spdlog/details/fmt_helper.h           |  54 ++++++++-
 include/spdlog/details/os-inl.h               |  16 ++-
 include/spdlog/details/tcp_client-windows.h   |   4 +-
 include/spdlog/details/tcp_client.h           |   3 +-
 include/spdlog/details/udp_client-windows.h   |   4 +-
 include/spdlog/fmt/bin_to_hex.h               |  12 +-
 include/spdlog/fmt/chrono.h                   |  16 +--
 include/spdlog/fmt/compile.h                  |  18 +--
 include/spdlog/fmt/fmt.h                      |   4 +-
 include/spdlog/fmt/ostr.h                     |  16 +--
 include/spdlog/fmt/xchar.h                    |  18 +--
 include/spdlog/logger.h                       | 103 ++++++++++++------
 include/spdlog/sinks/daily_file_sink.h        |   8 +-
 include/spdlog/sinks/hourly_file_sink.h       |   2 +-
 include/spdlog/sinks/msvc_sink.h              |   4 +
 include/spdlog/sinks/ringbuffer_sink.h        |   4 +
 include/spdlog/sinks/rotating_file_sink-inl.h |   2 +-
 include/spdlog/sinks/win_eventlog_sink.h      |  33 ++++--
 include/spdlog/spdlog.h                       |  32 +++---
 include/spdlog/tweakme.h                      |   7 ++
 src/fmt.cpp                                   |   2 +-
 tests/test_async.cpp                          |   2 +-
 tests/test_daily_logger.cpp                   |  30 ++++-
 tests/test_errors.cpp                         |   2 +-
 tests/test_file_helper.cpp                    |   2 +-
 tests/test_fmt_helper.cpp                     |  20 ++++
 tests/test_macros.cpp                         |   4 +-
 tests/test_pattern_formatter.cpp              |  62 ++++++++++-
 34 files changed, 445 insertions(+), 143 deletions(-)

diff --git a/CMakeLists.txt b/CMakeLists.txt
index 58e76f52..2d9bc279 100644
--- a/CMakeLists.txt
+++ b/CMakeLists.txt
@@ -80,6 +80,7 @@ option(SPDLOG_BUILD_WARNINGS "Enable compiler warnings" OFF)
 
 # install options
 option(SPDLOG_INSTALL "Generate the install target" ${SPDLOG_MASTER_PROJECT})
+option(SPDLOG_USE_STD_FORMAT "Use std::format instead of fmt library. No compile-time format string checking." OFF)
 option(SPDLOG_FMT_EXTERNAL "Use external fmt library instead of bundled" OFF)
 option(SPDLOG_FMT_EXTERNAL_HO "Use external fmt header-only library instead of bundled" OFF)
 option(SPDLOG_NO_EXCEPTIONS "Compile with -fno-exceptions. Call abort() on any spdlog exceptions" OFF)
@@ -88,6 +89,14 @@ if(SPDLOG_FMT_EXTERNAL AND SPDLOG_FMT_EXTERNAL_HO)
     message(FATAL_ERROR "SPDLOG_FMT_EXTERNAL and SPDLOG_FMT_EXTERNAL_HO are mutually exclusive")
 endif()
 
+if(SPDLOG_USE_STD_FORMAT AND SPDLOG_FMT_EXTERNAL_HO)
+    message(FATAL_ERROR "SPDLOG_USE_STD_FORMAT and SPDLOG_FMT_EXTERNAL_HO are mutually exclusive")
+endif()
+
+if(SPDLOG_USE_STD_FORMAT AND SPDLOG_FMT_EXTERNAL)
+    message(FATAL_ERROR "SPDLOG_USE_STD_FORMAT and SPDLOG_FMT_EXTERNAL are mutually exclusive")
+endif()
+
 # misc tweakme options
 if(WIN32)
     option(SPDLOG_WCHAR_SUPPORT "Support wchar api" OFF)
@@ -130,7 +139,7 @@ message(STATUS "Build type: " ${CMAKE_BUILD_TYPE})
 # ---------------------------------------------------------------------------------------
 set(SPDLOG_SRCS src/spdlog.cpp src/stdout_sinks.cpp src/color_sinks.cpp src/file_sinks.cpp src/async.cpp src/cfg.cpp)
 
-if(NOT SPDLOG_FMT_EXTERNAL AND NOT SPDLOG_FMT_EXTERNAL_HO)
+if(NOT SPDLOG_USE_STD_FORMAT AND NOT SPDLOG_FMT_EXTERNAL AND NOT SPDLOG_FMT_EXTERNAL_HO)
     list(APPEND SPDLOG_SRCS src/fmt.cpp)
 endif()
 
@@ -145,7 +154,7 @@ if(SPDLOG_BUILD_SHARED OR BUILD_SHARED_LIBS)
         target_compile_options(spdlog PUBLIC $<$<AND:$<CXX_COMPILER_ID:MSVC>,$<NOT:$<COMPILE_LANGUAGE:CUDA>>>:/wd4251
                                              /wd4275>)
     endif()
-    if(NOT SPDLOG_FMT_EXTERNAL AND NOT SPDLOG_FMT_EXTERNAL_HO)
+    if(NOT SPDLOG_USE_STD_FORMAT AND NOT SPDLOG_FMT_EXTERNAL AND NOT SPDLOG_FMT_EXTERNAL_HO)
         target_compile_definitions(spdlog PRIVATE FMT_EXPORT PUBLIC FMT_SHARED)
     endif()
 else()
@@ -280,7 +289,7 @@ if(SPDLOG_INSTALL)
         ARCHIVE DESTINATION ${CMAKE_INSTALL_LIBDIR}
         RUNTIME DESTINATION ${CMAKE_INSTALL_BINDIR})
 
-    if(NOT SPDLOG_FMT_EXTERNAL AND NOT SPDLOG_FMT_EXTERNAL_HO)
+    if(NOT SPDLOG_USE_STD_FORMAT AND NOT SPDLOG_FMT_EXTERNAL AND NOT SPDLOG_FMT_EXTERNAL_HO)
         install(DIRECTORY include/${PROJECT_NAME}/fmt/bundled/
                 DESTINATION "${CMAKE_INSTALL_INCLUDEDIR}/${PROJECT_NAME}/fmt/bundled/")
     endif()
diff --git a/appveyor.yml b/appveyor.yml
index b4574034..42d928e3 100644
--- a/appveyor.yml
+++ b/appveyor.yml
@@ -8,55 +8,72 @@ environment:
       WCHAR: 'OFF'
       WCHAR_FILES: 'OFF'
       BUILD_EXAMPLE: 'ON'
+      USE_STD_FORMAT: 'OFF'
     - GENERATOR: '"Visual Studio 14 2015"'
       BUILD_TYPE: Release
       BUILD_SHARED: 'OFF'
       WCHAR: 'ON'
       WCHAR_FILES: 'OFF'
       BUILD_EXAMPLE: 'ON'
+      USE_STD_FORMAT: 'OFF'
     - GENERATOR: '"Visual Studio 14 2015 Win64"'
       BUILD_TYPE: Debug
       BUILD_SHARED: 'OFF'
       WCHAR: 'ON'
       WCHAR_FILES: 'OFF'
       BUILD_EXAMPLE: 'ON'
+      USE_STD_FORMAT: 'OFF'
     - GENERATOR: '"Visual Studio 14 2015 Win64"'
       BUILD_TYPE: Release
       BUILD_SHARED: 'OFF'
       WCHAR: 'ON'
       WCHAR_FILES: 'OFF'
       BUILD_EXAMPLE: 'ON'
+      USE_STD_FORMAT: 'OFF'
     - GENERATOR: '"Visual Studio 15 2017 Win64"'
       BUILD_TYPE: Debug
       BUILD_SHARED: 'OFF'
       WCHAR: 'ON'
       WCHAR_FILES: 'OFF'
       BUILD_EXAMPLE: 'ON'
+      USE_STD_FORMAT: 'OFF'
     - GENERATOR: '"Visual Studio 15 2017 Win64"'
       BUILD_TYPE: Release
       BUILD_SHARED: 'OFF'
       WCHAR: 'OFF'
       WCHAR_FILES: 'OFF'
       BUILD_EXAMPLE: 'ON'
+      USE_STD_FORMAT: 'OFF'
     - GENERATOR: '"Visual Studio 15 2017 Win64"'
       BUILD_TYPE: Release
       BUILD_SHARED: 'ON'
       WCHAR: 'OFF'
       WCHAR_FILES: 'OFF'
       BUILD_EXAMPLE: 'ON'
+      USE_STD_FORMAT: 'OFF'
     - GENERATOR: '"Visual Studio 15 2017 Win64"'
       BUILD_TYPE: Release
       BUILD_SHARED: 'ON'
       WCHAR: 'ON'
       WCHAR_FILES: 'ON'
       BUILD_EXAMPLE: 'OFF'
+      USE_STD_FORMAT: 'OFF'
     - GENERATOR: '"Visual Studio 16 2019" -A x64'
       BUILD_TYPE: Release
       BUILD_SHARED: 'ON'
       WCHAR: 'OFF'
       WCHAR_FILES: 'OFF'
       BUILD_EXAMPLE: 'OFF'
+      USE_STD_FORMAT: 'OFF'
       APPVEYOR_BUILD_WORKER_IMAGE: Visual Studio 2019
+    - GENERATOR: '"Visual Studio 16 2022" -A x64'
+      BUILD_TYPE: Release
+      BUILD_SHARED: 'ON'
+      WCHAR: 'OFF'
+      WCHAR_FILES: 'OFF'
+      BUILD_EXAMPLE: 'OFF'
+      USE_STD_FORMAT: 'ON'
+      APPVEYOR_BUILD_WORKER_IMAGE: Visual Studio 2022
 build_script:
   - cmd: >-
       set
@@ -67,7 +84,7 @@ build_script:
 
       set PATH=%PATH%;C:\Program Files\Git\usr\bin
 
-      cmake -G %GENERATOR% -D CMAKE_BUILD_TYPE=%BUILD_TYPE% -D BUILD_SHARED_LIBS=%BUILD_SHARED% -D SPDLOG_WCHAR_SUPPORT=%WCHAR% -D SPDLOG_WCHAR_FILENAMES=%WCHAR_FILES% -D SPDLOG_BUILD_EXAMPLE=%BUILD_EXAMPLE% -D SPDLOG_BUILD_EXAMPLE_HO=%BUILD_EXAMPLE% -D SPDLOG_BUILD_TESTS=ON -D SPDLOG_BUILD_TESTS_HO=OFF -D SPDLOG_BUILD_WARNINGS=ON ..
+      cmake -G %GENERATOR% -D CMAKE_BUILD_TYPE=%BUILD_TYPE% -D BUILD_SHARED_LIBS=%BUILD_SHARED% -D SPDLOG_WCHAR_SUPPORT=%WCHAR% -D SPDLOG_WCHAR_FILENAMES=%WCHAR_FILES% -D SPDLOG_BUILD_EXAMPLE=%BUILD_EXAMPLE% -D SPDLOG_BUILD_EXAMPLE_HO=%BUILD_EXAMPLE% -D SPDLOG_BUILD_TESTS=ON -D SPDLOG_BUILD_TESTS_HO=OFF -D SPDLOG_BUILD_WARNINGS=ON -D SPDLOG_USE_STD_FORMAT=%USE_STD_FORMAT% ..
 
       cmake --build . --config %BUILD_TYPE%
 
diff --git a/bench/async_bench.cpp b/bench/async_bench.cpp
index 99d8ecb2..ce2c9281 100644
--- a/bench/async_bench.cpp
+++ b/bench/async_bench.cpp
@@ -10,7 +10,9 @@
 #include "spdlog/async.h"
 #include "spdlog/sinks/basic_file_sink.h"
 
-#ifdef SPDLOG_FMT_EXTERNAL
+#if defined(SPDLOG_USE_STD_FORMAT)
+#    include <format>
+#elif defined(SPDLOG_FMT_EXTERNAL)
 #    include <fmt/format.h>
 #else
 #    include "spdlog/fmt/bundled/format.h"
diff --git a/bench/bench.cpp b/bench/bench.cpp
index 5a4fc39b..af08c7ac 100644
--- a/bench/bench.cpp
+++ b/bench/bench.cpp
@@ -12,7 +12,9 @@
 #include "spdlog/sinks/null_sink.h"
 #include "spdlog/sinks/rotating_file_sink.h"
 
-#ifdef SPDLOG_FMT_EXTERNAL
+#if defined(SPDLOG_USE_STD_FORMAT)
+#    include <format>
+#elif defined(SPDLOG_FMT_EXTERNAL)
 #    include <fmt/locale.h>
 #else
 #    include "spdlog/fmt/bundled/format.h"
@@ -38,7 +40,7 @@ static const int max_threads = 1000;
 void bench_threaded_logging(size_t threads, int iters)
 {
     spdlog::info("**************************************************************");
-    spdlog::info(fmt::format(std::locale("en_US.UTF-8"), "Multi threaded: {:L} threads, {:L} messages", threads, iters));
+    spdlog::info(spdlog::fmt_lib::format(std::locale("en_US.UTF-8"), "Multi threaded: {:L} threads, {:L} messages", threads, iters));
     spdlog::info("**************************************************************");
 
     auto basic_mt = spdlog::basic_logger_mt("basic_mt", "logs/basic_mt.log", true);
@@ -74,7 +76,7 @@ void bench_threaded_logging(size_t threads, int iters)
 void bench_single_threaded(int iters)
 {
     spdlog::info("**************************************************************");
-    spdlog::info(fmt::format(std::locale("en_US.UTF-8"), "Single threaded: {} messages", iters));
+    spdlog::info(spdlog::fmt_lib::format(std::locale("en_US.UTF-8"), "Single threaded: {} messages", iters));
     spdlog::info("**************************************************************");
 
     auto basic_st = spdlog::basic_logger_st("basic_st", "logs/basic_st.log", true);
@@ -128,7 +130,7 @@ int main(int argc, char *argv[])
 
         if (threads > max_threads)
         {
-            throw std::runtime_error(fmt::format("Number of threads exceeds maximum({})", max_threads));
+            throw std::runtime_error(spdlog::fmt_lib::format("Number of threads exceeds maximum({})", max_threads));
         }
 
         bench_single_threaded(iters);
@@ -159,7 +161,7 @@ void bench(int howmany, std::shared_ptr<spdlog::logger> log)
     auto delta_d = duration_cast<duration<double>>(delta).count();
 
     spdlog::info(
-        fmt::format(std::locale("en_US.UTF-8"), "{:<30} Elapsed: {:0.2f} secs {:>16L}/sec", log->name(), delta_d, int(howmany / delta_d)));
+        spdlog::fmt_lib::format(std::locale("en_US.UTF-8"), "{:<30} Elapsed: {:0.2f} secs {:>16L}/sec", log->name(), delta_d, int(howmany / delta_d)));
     spdlog::drop(log->name());
 }
 
@@ -190,7 +192,7 @@ void bench_mt(int howmany, std::shared_ptr<spdlog::logger> log, size_t thread_co
     auto delta = high_resolution_clock::now() - start;
     auto delta_d = duration_cast<duration<double>>(delta).count();
     spdlog::info(
-        fmt::format(std::locale("en_US.UTF-8"), "{:<30} Elapsed: {:0.2f} secs {:>16L}/sec", log->name(), delta_d, int(howmany / delta_d)));
+        spdlog::fmt_lib::format(std::locale("en_US.UTF-8"), "{:<30} Elapsed: {:0.2f} secs {:>16L}/sec", log->name(), delta_d, int(howmany / delta_d)));
     spdlog::drop(log->name());
 }
 
diff --git a/include/spdlog/common-inl.h b/include/spdlog/common-inl.h
index 97cbbddc..9fd2bcb5 100644
--- a/include/spdlog/common-inl.h
+++ b/include/spdlog/common-inl.h
@@ -55,9 +55,13 @@ SPDLOG_INLINE spdlog_ex::spdlog_ex(std::string msg)
 
 SPDLOG_INLINE spdlog_ex::spdlog_ex(const std::string &msg, int last_errno)
 {
+#ifdef SPDLOG_USE_STD_FORMAT
+    msg_ = std::system_error(std::error_code(last_errno, std::generic_category()), msg).what();
+#else
     memory_buf_t outbuf;
     fmt::format_system_error(outbuf, last_errno, msg.c_str());
     msg_ = fmt::to_string(outbuf);
+#endif
 }
 
 SPDLOG_INLINE const char *spdlog_ex::what() const SPDLOG_NOEXCEPT
diff --git a/include/spdlog/common.h b/include/spdlog/common.h
index ddaa34ce..fb45d2a2 100644
--- a/include/spdlog/common.h
+++ b/include/spdlog/common.h
@@ -16,6 +16,10 @@
 #include <functional>
 #include <cstdio>
 
+#ifdef SPDLOG_USE_STD_FORMAT
+#include <string_view>
+#endif
+
 #ifdef SPDLOG_COMPILED_LIB
 #    undef SPDLOG_HEADER_ONLY
 #    if defined(_WIN32) && defined(SPDLOG_SHARED_LIB)
@@ -36,14 +40,15 @@
 
 #include <spdlog/fmt/fmt.h>
 
-// backward compatibility with fmt versions older than 8
-#if FMT_VERSION >= 80000
-#    define SPDLOG_FMT_RUNTIME(format_string) fmt::runtime(format_string)
-#    if defined(SPDLOG_WCHAR_FILENAMES) || defined(SPDLOG_WCHAR_TO_UTF8_SUPPORT)
-#        include <spdlog/fmt/xchar.h>
+#ifndef SPDLOG_USE_STD_FORMAT
+#    if FMT_VERSION >= 80000 // backward compatibility with fmt versions older than 8
+#        define SPDLOG_FMT_RUNTIME(format_string) fmt::runtime(format_string)
+#        if defined(SPDLOG_WCHAR_FILENAMES) || defined(SPDLOG_WCHAR_TO_UTF8_SUPPORT)
+#            include <spdlog/fmt/xchar.h>
+#        endif
+#    else
+#        define SPDLOG_FMT_RUNTIME(format_string) format_string
 #    endif
-#else
-#    define SPDLOG_FMT_RUNTIME(format_string) format_string
 #endif
 
 // visual studio upto 2013 does not support noexcept nor constexpr
@@ -112,11 +117,39 @@ using log_clock = std::chrono::system_clock;
 using sink_ptr = std::shared_ptr<sinks::sink>;
 using sinks_init_list = std::initializer_list<sink_ptr>;
 using err_handler = std::function<void(const std::string &err_msg)>;
+#ifdef SPDLOG_USE_STD_FORMAT
+namespace fmt_lib = std;
+
+using string_view_t = std::string_view;
+using wstring_view_t = std::wstring_view;
+using memory_buf_t = std::string;
+using wmemory_buf_t = std::wstring;
+
+template<typename... Args>
+using format_string_t = std::string_view;
+
+template<typename... Args>
+using wformat_string_t = std::wstring_view;
+
+template<class T, class Char = char>
+struct is_convertible_to_basic_format_string
+    : std::integral_constant<bool,
+          std::is_convertible<T, std::basic_string_view<Char>>::value>
+{};
+#else
+namespace fmt_lib = fmt;
+
 using string_view_t = fmt::basic_string_view<char>;
 using wstring_view_t = fmt::basic_string_view<wchar_t>;
 using memory_buf_t = fmt::basic_memory_buffer<char, 250>;
 using wmemory_buf_t = fmt::basic_memory_buffer<wchar_t, 250>;
 
+template<typename... Args>
+using format_string_t = fmt::format_string<Args...>;
+
+template<typename... Args>
+using wformat_string_t = fmt::wformat_string<Args...>;
+
 template<class T>
 using remove_cvref_t = typename std::remove_cv<typename std::remove_reference<T>::type>::type;
 
@@ -127,6 +160,7 @@ struct is_convertible_to_basic_format_string
     : std::integral_constant<bool,
           std::is_convertible<T, fmt::basic_string_view<Char>>::value || std::is_same<remove_cvref_t<T>, fmt::basic_runtime<Char>>::value>
 {};
+#endif
 
 #ifdef SPDLOG_WCHAR_TO_UTF8_SUPPORT
 #    ifndef _WIN32
diff --git a/include/spdlog/details/fmt_helper.h b/include/spdlog/details/fmt_helper.h
index 01b2aa4d..31ed64fd 100644
--- a/include/spdlog/details/fmt_helper.h
+++ b/include/spdlog/details/fmt_helper.h
@@ -8,6 +8,11 @@
 #include <spdlog/fmt/fmt.h>
 #include <spdlog/common.h>
 
+#ifdef SPDLOG_USE_STD_FORMAT
+#include <charconv>
+#include <limits>
+#endif
+
 // Some fmt helpers to efficiently format and pad ints and strings
 namespace spdlog {
 namespace details {
@@ -24,17 +29,63 @@ inline void append_string_view(spdlog::string_view_t view, memory_buf_t &dest)
     dest.append(buf_ptr, buf_ptr + view.size());
 }
 
+#ifdef SPDLOG_USE_STD_FORMAT
+template<typename T>
+inline void append_int(T n, memory_buf_t &dest)
+{
+    // Buffer should be large enough to hold all digits (digits10 + 1) and a sign
+    SPDLOG_CONSTEXPR auto BUF_SIZE = std::numeric_limits<T>::digits10 + 2;
+    char buf[BUF_SIZE];
+
+    auto [ptr, ec] = std::to_chars(buf, buf + BUF_SIZE, n, 10);
+    if (ec == std::errc())
+    {
+        dest.append(buf, ptr);
+    }
+    else
+    {
+        throw_spdlog_ex("Failed to format int", static_cast<int>(ec));
+    }
+}
+#else
 template<typename T>
 inline void append_int(T n, memory_buf_t &dest)
 {
     fmt::format_int i(n);
     dest.append(i.data(), i.data() + i.size());
 }
+#endif
+
+template<typename T>
+SPDLOG_CONSTEXPR unsigned int count_digits_fallback(T n)
+{
+    // taken from fmt.
+    unsigned int count = 1;
+    for (;;)
+    {
+        // Integer division is slow so do it for a group of four digits instead
+        // of for every digit. The idea comes from the talk by Alexandrescu
+        // "Three Optimization Tips for C++". See speed-test for a comparison.
+        if (n < 10)
+            return count;
+        if (n < 100)
+            return count + 1;
+        if (n < 1000)
+            return count + 2;
+        if (n < 10000)
+            return count + 3;
+        n /= 10000u;
+        count += 4;
+    }
+}
 
 template<typename T>
 inline unsigned int count_digits(T n)
 {
     using count_type = typename std::conditional<(sizeof(T) > sizeof(uint32_t)), uint64_t, uint32_t>::type;
+#ifdef SPDLOG_USE_STD_FORMAT
+    return count_digits_fallback(static_cast<count_type>(n));
+#else
     return static_cast<unsigned int>(fmt::
 // fmt 7.0.0 renamed the internal namespace to detail.
 // See: https://github.com/fmtlib/fmt/issues/1538
@@ -44,6 +95,7 @@ inline unsigned int count_digits(T n)
             detail
 #endif
         ::count_digits(static_cast<count_type>(n)));
+#endif
 }
 
 inline void pad2(int n, memory_buf_t &dest)
@@ -55,7 +107,7 @@ inline void pad2(int n, memory_buf_t &dest)
     }
     else // unlikely, but just in case, let fmt deal with it
     {
-        fmt::format_to(std::back_inserter(dest), SPDLOG_FMT_RUNTIME("{:02}"), n);
+        fmt_lib::format_to(std::back_inserter(dest), "{:02}", n);
     }
 }
 
diff --git a/include/spdlog/details/os-inl.h b/include/spdlog/details/os-inl.h
index 4602782c..9b2ef0d8 100644
--- a/include/spdlog/details/os-inl.h
+++ b/include/spdlog/details/os-inl.h
@@ -381,7 +381,11 @@ SPDLOG_INLINE std::string filename_to_str(const filename_t &filename)
 {
     memory_buf_t buf;
     wstr_to_utf8buf(filename, buf);
+#ifdef SPDLOG_USE_STD_FORMAT
+    return buf;
+#else
     return fmt::to_string(buf);
+#endif
 }
 #else
 SPDLOG_INLINE std::string filename_to_str(const filename_t &filename)
@@ -458,8 +462,12 @@ SPDLOG_INLINE void wstr_to_utf8buf(wstring_view_t wstr, memory_buf_t &target)
         return;
     }
 
+#ifdef SPDLOG_USE_STD_FORMAT
+    int result_size = 0;
+#else
     int result_size = static_cast<int>(target.capacity());
     if ((wstr_size + 1) * 2 > result_size)
+#endif
     {
         result_size = ::WideCharToMultiByte(CP_UTF8, 0, wstr.data(), wstr_size, NULL, 0, NULL, NULL);
     }
@@ -476,7 +484,7 @@ SPDLOG_INLINE void wstr_to_utf8buf(wstring_view_t wstr, memory_buf_t &target)
         }
     }
 
-    throw_spdlog_ex(fmt::format("WideCharToMultiByte failed. Last error: {}", ::GetLastError()));
+    throw_spdlog_ex(fmt_lib::format("WideCharToMultiByte failed. Last error: {}", ::GetLastError()));
 }
 
 SPDLOG_INLINE void utf8_to_wstrbuf(string_view_t str, wmemory_buf_t &target)
@@ -493,8 +501,12 @@ SPDLOG_INLINE void utf8_to_wstrbuf(string_view_t str, wmemory_buf_t &target)
         return;
     }
 
+#ifdef SPDLOG_USE_STD_FORMAT
+    int result_size = 0;
+#else
     int result_size = static_cast<int>(target.capacity());
     if (str_size + 1 > result_size)
+#endif
     {
         result_size = ::MultiByteToWideChar(CP_UTF8, MB_ERR_INVALID_CHARS, str.data(), str_size, NULL, 0);
     }
@@ -511,7 +523,7 @@ SPDLOG_INLINE void utf8_to_wstrbuf(string_view_t str, wmemory_buf_t &target)
         }
     }
 
-    throw_spdlog_ex(fmt::format("MultiByteToWideChar failed. Last error: {}", ::GetLastError()));
+    throw_spdlog_ex(fmt_lib::format("MultiByteToWideChar failed. Last error: {}", ::GetLastError()));
 }
 #endif // (defined(SPDLOG_WCHAR_TO_UTF8_SUPPORT) || defined(SPDLOG_WCHAR_FILENAMES)) && defined(_WIN32)
 
diff --git a/include/spdlog/details/tcp_client-windows.h b/include/spdlog/details/tcp_client-windows.h
index 45b69630..b331a500 100644
--- a/include/spdlog/details/tcp_client-windows.h
+++ b/include/spdlog/details/tcp_client-windows.h
@@ -38,10 +38,10 @@ class tcp_client
     static void throw_winsock_error_(const std::string &msg, int last_error)
     {
         char buf[512];
-        ::FormatMessage(FORMAT_MESSAGE_FROM_SYSTEM | FORMAT_MESSAGE_IGNORE_INSERTS, NULL, last_error,
+        ::FormatMessageA(FORMAT_MESSAGE_FROM_SYSTEM | FORMAT_MESSAGE_IGNORE_INSERTS, NULL, last_error,
             MAKELANGID(LANG_NEUTRAL, SUBLANG_DEFAULT), buf, (sizeof(buf) / sizeof(char)), NULL);
 
-        throw_spdlog_ex(fmt::format("tcp_sink - {}: {}", msg, buf));
+        throw_spdlog_ex(fmt_lib::format("tcp_sink - {}: {}", msg, buf));
     }
 
 public:
diff --git a/include/spdlog/details/tcp_client.h b/include/spdlog/details/tcp_client.h
index 706d7599..0daff0eb 100644
--- a/include/spdlog/details/tcp_client.h
+++ b/include/spdlog/details/tcp_client.h
@@ -67,8 +67,7 @@ public:
         auto rv = ::getaddrinfo(host.c_str(), port_str.c_str(), &hints, &addrinfo_result);
         if (rv != 0)
         {
-            auto msg = fmt::format("::getaddrinfo failed: {}", gai_strerror(rv));
-            throw_spdlog_ex(msg);
+            throw_spdlog_ex(fmt_lib::format("::getaddrinfo failed: {}", gai_strerror(rv)));
         }
 
         // Try each address until we successfully connect(2).
diff --git a/include/spdlog/details/udp_client-windows.h b/include/spdlog/details/udp_client-windows.h
index 5cc3dd88..372efa6d 100644
--- a/include/spdlog/details/udp_client-windows.h
+++ b/include/spdlog/details/udp_client-windows.h
@@ -40,10 +40,10 @@ class udp_client
     static void throw_winsock_error_(const std::string &msg, int last_error)
     {
         char buf[512];
-        ::FormatMessage(FORMAT_MESSAGE_FROM_SYSTEM | FORMAT_MESSAGE_IGNORE_INSERTS, NULL, last_error,
+        ::FormatMessageA(FORMAT_MESSAGE_FROM_SYSTEM | FORMAT_MESSAGE_IGNORE_INSERTS, NULL, last_error,
             MAKELANGID(LANG_NEUTRAL, SUBLANG_DEFAULT), buf, (sizeof(buf) / sizeof(char)), NULL);
 
-        throw_spdlog_ex(fmt::format("udp_sink - {}: {}", msg, buf));
+        throw_spdlog_ex(fmt_lib::format("udp_sink - {}: {}", msg, buf));
     }
 
     void cleanup_()
diff --git a/include/spdlog/fmt/bin_to_hex.h b/include/spdlog/fmt/bin_to_hex.h
index 0b6c3e96..62807d63 100644
--- a/include/spdlog/fmt/bin_to_hex.h
+++ b/include/spdlog/fmt/bin_to_hex.h
@@ -74,12 +74,8 @@ inline details::dump_info<It> to_hex(const It range_begin, const It range_end, s
     return details::dump_info<It>(range_begin, range_end, size_per_line);
 }
 
-} // namespace spdlog
-
-namespace fmt {
-
 template<typename T>
-struct formatter<spdlog::details::dump_info<T>>
+struct fmt_lib::formatter<spdlog::details::dump_info<T>, char>
 {
     const char delimiter = ' ';
     bool put_newlines = true;
@@ -90,7 +86,7 @@ struct formatter<spdlog::details::dump_info<T>>
 
     // parse the format string flags
     template<typename ParseContext>
-    FMT_CONSTEXPR auto parse(ParseContext &ctx) -> decltype(ctx.begin())
+    SPDLOG_CONSTEXPR auto parse(ParseContext &ctx) -> decltype(ctx.begin())
     {
         auto it = ctx.begin();
         while (it != ctx.end() && *it != '}')
@@ -210,8 +206,8 @@ struct formatter<spdlog::details::dump_info<T>>
 
         if (put_positions)
         {
-            fmt::format_to(inserter, "{:04X}: ", pos);
+            fmt_lib::format_to(inserter, "{:04X}: ", pos);
         }
     }
 };
-} // namespace fmt
+} // namespace spdlog
diff --git a/include/spdlog/fmt/chrono.h b/include/spdlog/fmt/chrono.h
index a0544bca..83fad2ff 100644
--- a/include/spdlog/fmt/chrono.h
+++ b/include/spdlog/fmt/chrono.h
@@ -8,13 +8,15 @@
 // include bundled or external copy of fmtlib's chrono support
 //
 
-#if !defined(SPDLOG_FMT_EXTERNAL)
-#    ifdef SPDLOG_HEADER_ONLY
-#        ifndef FMT_HEADER_ONLY
-#            define FMT_HEADER_ONLY
+#if !defined(SPDLOG_USE_STD_FORMAT)
+#    if !defined(SPDLOG_FMT_EXTERNAL)
+#        ifdef SPDLOG_HEADER_ONLY
+#            ifndef FMT_HEADER_ONLY
+#                define FMT_HEADER_ONLY
+#            endif
 #        endif
+#        include <spdlog/fmt/bundled/chrono.h>
+#    else
+#        include <fmt/chrono.h>
 #    endif
-#    include <spdlog/fmt/bundled/chrono.h>
-#else
-#    include <fmt/chrono.h>
 #endif
diff --git a/include/spdlog/fmt/compile.h b/include/spdlog/fmt/compile.h
index 6f345935..906e9f57 100644
--- a/include/spdlog/fmt/compile.h
+++ b/include/spdlog/fmt/compile.h
@@ -5,16 +5,18 @@
 
 #pragma once
 //
-// include bundled or external copy of fmtlib's ostream support
+// include bundled or external copy of fmtlib's compile-time support
 //
 
-#if !defined(SPDLOG_FMT_EXTERNAL)
-#    ifdef SPDLOG_HEADER_ONLY
-#        ifndef FMT_HEADER_ONLY
-#            define FMT_HEADER_ONLY
+#if !defined(SPDLOG_USE_STD_FORMAT)
+#    if !defined(SPDLOG_FMT_EXTERNAL)
+#        ifdef SPDLOG_HEADER_ONLY
+#            ifndef FMT_HEADER_ONLY
+#                define FMT_HEADER_ONLY
+#            endif
 #        endif
+#        include <spdlog/fmt/bundled/compile.h>
+#    else
+#        include <fmt/compile.h>
 #    endif
-#    include <spdlog/fmt/bundled/compile.h>
-#else
-#    include <fmt/compile.h>
 #endif
diff --git a/include/spdlog/fmt/fmt.h b/include/spdlog/fmt/fmt.h
index 7aaeed81..fa4a2a84 100644
--- a/include/spdlog/fmt/fmt.h
+++ b/include/spdlog/fmt/fmt.h
@@ -10,7 +10,9 @@
 // By default spdlog include its own copy.
 //
 
-#if !defined(SPDLOG_FMT_EXTERNAL)
+#if defined(SPDLOG_USE_STD_FORMAT) // SPDLOG_USE_STD_FORMAT is defined - use std::format
+#    include <format>
+#elif !defined(SPDLOG_FMT_EXTERNAL)
 #    if !defined(SPDLOG_COMPILED_LIB) && !defined(FMT_HEADER_ONLY)
 #        define FMT_HEADER_ONLY
 #    endif
diff --git a/include/spdlog/fmt/ostr.h b/include/spdlog/fmt/ostr.h
index 9d8efe38..75880341 100644
--- a/include/spdlog/fmt/ostr.h
+++ b/include/spdlog/fmt/ostr.h
@@ -8,13 +8,15 @@
 // include bundled or external copy of fmtlib's ostream support
 //
 
-#if !defined(SPDLOG_FMT_EXTERNAL)
-#    ifdef SPDLOG_HEADER_ONLY
-#        ifndef FMT_HEADER_ONLY
-#            define FMT_HEADER_ONLY
+#if !defined(SPDLOG_USE_STD_FORMAT)
+#    if !defined(SPDLOG_FMT_EXTERNAL)
+#        ifdef SPDLOG_HEADER_ONLY
+#            ifndef FMT_HEADER_ONLY
+#                define FMT_HEADER_ONLY
+#            endif
 #        endif
+#        include <spdlog/fmt/bundled/ostream.h>
+#    else
+#        include <fmt/ostream.h>
 #    endif
-#    include <spdlog/fmt/bundled/ostream.h>
-#else
-#    include <fmt/ostream.h>
 #endif
diff --git a/include/spdlog/fmt/xchar.h b/include/spdlog/fmt/xchar.h
index 616d8f5b..9a766e5a 100644
--- a/include/spdlog/fmt/xchar.h
+++ b/include/spdlog/fmt/xchar.h
@@ -5,16 +5,18 @@
 
 #pragma once
 //
-// include bundled or external copy of fmtlib's ostream support
+// include bundled or external copy of fmtlib's xchar support
 //
 
-#if !defined(SPDLOG_FMT_EXTERNAL)
-#    ifdef SPDLOG_HEADER_ONLY
-#        ifndef FMT_HEADER_ONLY
-#            define FMT_HEADER_ONLY
+#if !defined(SPDLOG_USE_STD_FORMAT)
+#    if !defined(SPDLOG_FMT_EXTERNAL)
+#        ifdef SPDLOG_HEADER_ONLY
+#            ifndef FMT_HEADER_ONLY
+#                define FMT_HEADER_ONLY
+#            endif
 #        endif
+#        include <spdlog/fmt/bundled/xchar.h>
+#    else
+#        include <fmt/xchar.h>
 #    endif
-#    include <spdlog/fmt/bundled/xchar.h>
-#else
-#    include <fmt/xchar.h>
 #endif
diff --git a/include/spdlog/logger.h b/include/spdlog/logger.h
index eea0afc2..9adfec97 100644
--- a/include/spdlog/logger.h
+++ b/include/spdlog/logger.h
@@ -33,7 +33,7 @@
         {                                                                                                                                  \
             if(location.filename)                                                                                                          \
             {                                                                                                                              \
-                err_handler_(fmt::format("{} [{}({})]", ex.what(), location.filename, location.line));                                     \
+                err_handler_(fmt_lib::format("{} [{}({})]", ex.what(), location.filename, location.line));                                     \
             }                                                                                                                              \
             else                                                                                                                           \
             {                                                                                                                              \
@@ -85,13 +85,13 @@ public:
     void swap(spdlog::logger &other) SPDLOG_NOEXCEPT;
 
     template<typename... Args>
-    void log(source_loc loc, level::level_enum lvl, fmt::format_string<Args...> fmt, Args &&...args)
+    void log(source_loc loc, level::level_enum lvl, format_string_t<Args...> fmt, Args &&...args)
     {
         log_(loc, lvl, fmt, std::forward<Args>(args)...);
     }
 
     template<typename... Args>
-    void log(level::level_enum lvl, fmt::format_string<Args...> fmt, Args &&...args)
+    void log(level::level_enum lvl, format_string_t<Args...> fmt, Args &&...args)
     {
         log(source_loc{}, lvl, fmt, std::forward<Args>(args)...);
     }
@@ -102,14 +102,7 @@ public:
         log(source_loc{}, lvl, msg);
     }
 
-    // T can be statically converted to string_view
-    template<class T, typename std::enable_if<std::is_convertible<const T &, spdlog::string_view_t>::value, int>::type = 0>
-    void log(source_loc loc, level::level_enum lvl, const T &msg)
-    {
-        log(loc, lvl, string_view_t{msg});
-    }
-
-    // T cannot be statically converted to format string (including string_view)
+    // T cannot be statically converted to format string (including string_view/wstring_view)
     template<class T, typename std::enable_if<!is_convertible_to_any_format_string<const T &>::value, int>::type = 0>
     void log(source_loc loc, level::level_enum lvl, const T &msg)
     {
@@ -148,86 +141,121 @@ public:
     }
 
     template<typename... Args>
-    void trace(fmt::format_string<Args...> fmt, Args &&...args)
+    void trace(format_string_t<Args...> fmt, Args &&...args)
     {
         log(level::trace, fmt, std::forward<Args>(args)...);
     }
 
     template<typename... Args>
-    void debug(fmt::format_string<Args...> fmt, Args &&...args)
+    void debug(format_string_t<Args...> fmt, Args &&...args)
     {
         log(level::debug, fmt, std::forward<Args>(args)...);
     }
 
     template<typename... Args>
-    void info(fmt::format_string<Args...> fmt, Args &&...args)
+    void info(format_string_t<Args...> fmt, Args &&...args)
     {
         log(level::info, fmt, std::forward<Args>(args)...);
     }
 
     template<typename... Args>
-    void warn(fmt::format_string<Args...> fmt, Args &&...args)
+    void warn(format_string_t<Args...> fmt, Args &&...args)
     {
         log(level::warn, fmt, std::forward<Args>(args)...);
     }
 
     template<typename... Args>
-    void error(fmt::format_string<Args...> fmt, Args &&...args)
+    void error(format_string_t<Args...> fmt, Args &&...args)
     {
         log(level::err, fmt, std::forward<Args>(args)...);
     }
 
     template<typename... Args>
-    void critical(fmt::format_string<Args...> fmt, Args &&...args)
+    void critical(format_string_t<Args...> fmt, Args &&...args)
     {
         log(level::critical, fmt, std::forward<Args>(args)...);
     }
 
 #ifdef SPDLOG_WCHAR_TO_UTF8_SUPPORT
     template<typename... Args>
-    void log(level::level_enum lvl, fmt::wformat_string<Args...> fmt, Args &&...args)
-    {
-        log(source_loc{}, lvl, fmt, std::forward<Args>(args)...);
-    }
-
-    template<typename... Args>
-    void log(source_loc loc, level::level_enum lvl, fmt::wformat_string<Args...> fmt, Args &&...args)
+    void log(source_loc loc, level::level_enum lvl, wformat_string_t<Args...> fmt, Args &&...args)
     {
         log_(loc, lvl, fmt, std::forward<Args>(args)...);
     }
 
     template<typename... Args>
-    void trace(fmt::wformat_string<Args...> fmt, Args &&...args)
+    void log(level::level_enum lvl, wformat_string_t<Args...> fmt, Args &&...args)
+    {
+        log(source_loc{}, lvl, fmt, std::forward<Args>(args)...);
+    }
+
+    void log(log_clock::time_point log_time, source_loc loc, level::level_enum lvl, wstring_view_t msg)
+    {
+        bool log_enabled = should_log(lvl);
+        bool traceback_enabled = tracer_.enabled();
+        if (!log_enabled && !traceback_enabled)
+        {
+            return;
+        }
+
+        memory_buf_t buf;
+        details::os::wstr_to_utf8buf(wstring_view_t(msg.data(), msg.size()), buf);
+        details::log_msg log_msg(log_time, loc, name_, lvl, string_view_t(buf.data(), buf.size()));
+        log_it_(log_msg, log_enabled, traceback_enabled);
+    }
+
+    void log(source_loc loc, level::level_enum lvl, wstring_view_t msg)
+    {
+        bool log_enabled = should_log(lvl);
+        bool traceback_enabled = tracer_.enabled();
+        if (!log_enabled && !traceback_enabled)
+        {
+            return;
+        }
+
+        memory_buf_t buf;
+        details::os::wstr_to_utf8buf(wstring_view_t(msg.data(), msg.size()), buf);
+        details::log_msg log_msg(loc, name_, lvl, string_view_t(buf.data(), buf.size()));
+        log_it_(log_msg, log_enabled, traceback_enabled);
+    }
+
+    void log(level::level_enum lvl, wstring_view_t msg)
+    {
+        log(source_loc{}, lvl, msg);
+    }
+
+    template<typename... Args>
+    void trace(wformat_string_t<Args...> fmt, Args &&...args)
     {
         log(level::trace, fmt, std::forward<Args>(args)...);
     }
 
     template<typename... Args>
-    void debug(fmt::wformat_string<Args...> fmt, Args &&...args)
+    void debug(wformat_string_t<Args...> fmt, Args &&...args)
     {
         log(level::debug, fmt, std::forward<Args>(args)...);
     }
 
     template<typename... Args>
-    void info(fmt::wformat_string<Args...> fmt, Args &&...args)
+    void info(wformat_string_t<Args...> fmt, Args &&...args)
     {
         log(level::info, fmt, std::forward<Args>(args)...);
     }
 
     template<typename... Args>
-    void warn(fmt::wformat_string<Args...> fmt, Args &&...args)
+    void warn(wformat_string_t<Args...> fmt, Args &&...args)
     {
         log(level::warn, fmt, std::forward<Args>(args)...);
     }
 
     template<typename... Args>
-    void error(fmt::wformat_string<Args...> fmt, Args &&...args)
+    void error(wformat_string_t<Args...> fmt, Args &&...args)
     {
         log(level::err, fmt, std::forward<Args>(args)...);
     }
 
     template<typename... Args>
-    void critical(fmt::wformat_string<Args...> fmt, Args &&...args)
+    void critical(wformat_string_t<Args...> fmt, Args &&...args)
     {
         log(level::critical, fmt, std::forward<Args>(args)...);
     }
@@ -335,8 +363,12 @@ protected:
         }
         SPDLOG_TRY
         {
+#ifdef SPDLOG_USE_STD_FORMAT
+            memory_buf_t buf = std::vformat(fmt, std::make_format_args(std::forward<Args...>(args)...));
+#else
             memory_buf_t buf;
-            fmt::detail::vformat_to(buf, fmt, fmt::make_format_args(args...));
+            fmt::detail::vformat_to(buf, fmt, fmt::make_format_args(std::forward<Args...>(args)...));
+#endif
             details::log_msg log_msg(loc, name_, lvl, string_view_t(buf.data(), buf.size()));
             log_it_(log_msg, log_enabled, traceback_enabled);
         }
@@ -356,8 +388,13 @@ protected:
         SPDLOG_TRY
         {
             // format to wmemory_buffer and convert to utf8
-            fmt::wmemory_buffer wbuf;
-            fmt::detail::vformat_to(wbuf, fmt, fmt::make_format_args<fmt::wformat_context>(args...));
+            ;
+#ifdef SPDLOG_USE_STD_FORMAT
+            wmemory_buf_t wbuf = std::vformat(fmt, std::make_wformat_args(std::forward<Args...>(args)...));
+#else
+            wmemory_buf_t wbuf;
+            fmt::detail::vformat_to(wbuf, fmt, fmt::make_format_args<fmt::wformat_context>(std::forward<Args...>(args)...));
+#endif
             memory_buf_t buf;
             details::os::wstr_to_utf8buf(wstring_view_t(wbuf.data(), wbuf.size()), buf);
             details::log_msg log_msg(loc, name_, lvl, string_view_t(buf.data(), buf.size()));
diff --git a/include/spdlog/sinks/daily_file_sink.h b/include/spdlog/sinks/daily_file_sink.h
index fc552398..cf83800a 100644
--- a/include/spdlog/sinks/daily_file_sink.h
+++ b/include/spdlog/sinks/daily_file_sink.h
@@ -32,7 +32,7 @@ struct daily_filename_calculator
     {
         filename_t basename, ext;
         std::tie(basename, ext) = details::file_helper::split_by_extension(filename);
-        return fmt::format(
+        return fmt_lib::format(
             SPDLOG_FILENAME_T("{}_{:04d}-{:02d}-{:02d}{}"), basename, now_tm.tm_year + 1900, now_tm.tm_mon + 1, now_tm.tm_mday, ext);
     }
 };
@@ -49,8 +49,10 @@ struct daily_filename_format_calculator
     static filename_t calc_filename(const filename_t &filename, const tm &now_tm)
     {
         // generate fmt datetime format string, e.g. {:%Y-%m-%d}.
-        filename_t fmt_filename = fmt::format(SPDLOG_FILENAME_T("{{:{}}}"), filename);
-#if defined(_MSC_VER) && defined(SPDLOG_WCHAR_FILENAMES) // for some reason msvc doesnt allow fmt::runtime(..) with wchar here
+        filename_t fmt_filename = fmt_lib::format(SPDLOG_FILENAME_T("{{:{}}}"), filename);
+#if defined(SPDLOG_USE_STD_FORMAT)
+        return std::vformat(fmt_filename, std::make_format_args(now_tm));
+#elif defined(_MSC_VER) && defined(SPDLOG_WCHAR_FILENAMES) // for some reason msvc doesnt allow fmt::runtime(..) with wchar here
         return fmt::format(fmt_filename, now_tm);
 #else
         return fmt::format(SPDLOG_FMT_RUNTIME(fmt_filename), now_tm);
diff --git a/include/spdlog/sinks/hourly_file_sink.h b/include/spdlog/sinks/hourly_file_sink.h
index 653faceb..71b5a1b4 100644
--- a/include/spdlog/sinks/hourly_file_sink.h
+++ b/include/spdlog/sinks/hourly_file_sink.h
@@ -31,7 +31,7 @@ struct hourly_filename_calculator
     {
         filename_t basename, ext;
         std::tie(basename, ext) = details::file_helper::split_by_extension(filename);
-        return fmt::format(SPDLOG_FILENAME_T("{}_{:04d}-{:02d}-{:02d}_{:02d}{}"), basename, now_tm.tm_year + 1900, now_tm.tm_mon + 1,
+        return fmt_lib::format(SPDLOG_FILENAME_T("{}_{:04d}-{:02d}-{:02d}_{:02d}{}"), basename, now_tm.tm_year + 1900, now_tm.tm_mon + 1,
             now_tm.tm_mday, now_tm.tm_hour, ext);
     }
 };
diff --git a/include/spdlog/sinks/msvc_sink.h b/include/spdlog/sinks/msvc_sink.h
index 2ac4a965..76ec40a0 100644
--- a/include/spdlog/sinks/msvc_sink.h
+++ b/include/spdlog/sinks/msvc_sink.h
@@ -30,7 +30,11 @@ protected:
     {
         memory_buf_t formatted;
         base_sink<Mutex>::formatter_->format(msg, formatted);
+#ifdef SPDLOG_USE_STD_FORMAT
+        OutputDebugStringA(formatted.c_str());
+#else
         OutputDebugStringA(fmt::to_string(formatted).c_str());
+#endif
     }
 
     void flush_() override {}
diff --git a/include/spdlog/sinks/ringbuffer_sink.h b/include/spdlog/sinks/ringbuffer_sink.h
index 1ee3f691..5836d98b 100644
--- a/include/spdlog/sinks/ringbuffer_sink.h
+++ b/include/spdlog/sinks/ringbuffer_sink.h
@@ -50,7 +50,11 @@ public:
         {
             memory_buf_t formatted;
             base_sink<Mutex>::formatter_->format(q_.at(i), formatted);
+#ifdef SPDLOG_USE_STD_FORMAT
+            ret.push_back(std::move(formatted));
+#else
             ret.push_back(fmt::to_string(formatted));
+#endif
         }
         return ret;
     }
diff --git a/include/spdlog/sinks/rotating_file_sink-inl.h b/include/spdlog/sinks/rotating_file_sink-inl.h
index 2e3087af..87e80593 100644
--- a/include/spdlog/sinks/rotating_file_sink-inl.h
+++ b/include/spdlog/sinks/rotating_file_sink-inl.h
@@ -51,7 +51,7 @@ SPDLOG_INLINE filename_t rotating_file_sink<Mutex>::calc_filename(const filename
 
     filename_t basename, ext;
     std::tie(basename, ext) = details::file_helper::split_by_extension(filename);
-    return fmt::format(SPDLOG_FILENAME_T("{}.{}{}"), basename, index, ext);
+    return fmt_lib::format(SPDLOG_FILENAME_T("{}.{}{}"), basename, index, ext);
 }
 
 template<typename Mutex>
diff --git a/include/spdlog/sinks/win_eventlog_sink.h b/include/spdlog/sinks/win_eventlog_sink.h
index 5e70599a..202c79f8 100644
--- a/include/spdlog/sinks/win_eventlog_sink.h
+++ b/include/spdlog/sinks/win_eventlog_sink.h
@@ -47,6 +47,24 @@ namespace win_eventlog {
 
 namespace internal {
 
+struct local_alloc_t
+{
+    HLOCAL hlocal_;
+
+    SPDLOG_CONSTEXPR local_alloc_t() SPDLOG_NOEXCEPT : hlocal_(nullptr) {}
+
+    local_alloc_t(local_alloc_t const&) = delete;
+    local_alloc_t& operator=(local_alloc_t const&) = delete;
+
+    ~local_alloc_t() SPDLOG_NOEXCEPT
+    {
+        if (hlocal_)
+        {
+            LocalFree(hlocal_);
+        }
+    }
+};
+
 /** Windows error */
 struct win32_error : public spdlog_ex
 {
@@ -55,22 +73,17 @@ struct win32_error : public spdlog_ex
     {
         std::string system_message;
 
-        LPSTR format_message_result{};
+        local_alloc_t format_message_result{};
         auto format_message_succeeded =
             ::FormatMessageA(FORMAT_MESSAGE_ALLOCATE_BUFFER | FORMAT_MESSAGE_FROM_SYSTEM | FORMAT_MESSAGE_IGNORE_INSERTS, nullptr,
-                error_code, MAKELANGID(LANG_NEUTRAL, SUBLANG_DEFAULT), (LPSTR)&format_message_result, 0, nullptr);
+                error_code, MAKELANGID(LANG_NEUTRAL, SUBLANG_DEFAULT), (LPSTR)&format_message_result.hlocal_, 0, nullptr);
 
-        if (format_message_succeeded && format_message_result)
+        if (format_message_succeeded && format_message_result.hlocal_)
         {
-            system_message = fmt::format(" ({})", format_message_result);
+            system_message = fmt_lib::format(" ({})", (LPSTR)format_message_result.hlocal_);
         }
 
-        if (format_message_result)
-        {
-            LocalFree((HLOCAL)format_message_result);
-        }
-
-        return fmt::format("{}: {}{}", user_message, error_code, system_message);
+        return fmt_lib::format("{}: {}{}", user_message, error_code, system_message);
     }
 
     explicit win32_error(std::string const &func_name, DWORD error = GetLastError())
diff --git a/include/spdlog/spdlog.h b/include/spdlog/spdlog.h
index 812896da..327a0ebe 100644
--- a/include/spdlog/spdlog.h
+++ b/include/spdlog/spdlog.h
@@ -128,49 +128,49 @@ SPDLOG_API spdlog::logger *default_logger_raw();
 SPDLOG_API void set_default_logger(std::shared_ptr<spdlog::logger> default_logger);
 
 template<typename... Args>
-inline void log(source_loc source, level::level_enum lvl, fmt::format_string<Args...> fmt, Args &&...args)
+inline void log(source_loc source, level::level_enum lvl, format_string_t<Args...> fmt, Args &&...args)
 {
     default_logger_raw()->log(source, lvl, fmt, std::forward<Args>(args)...);
 }
 
 template<typename... Args>
-inline void log(level::level_enum lvl, fmt::format_string<Args...> fmt, Args &&...args)
+inline void log(level::level_enum lvl, format_string_t<Args...> fmt, Args &&...args)
 {
     default_logger_raw()->log(source_loc{}, lvl, fmt, std::forward<Args>(args)...);
 }
 
 template<typename... Args>
-inline void trace(fmt::format_string<Args...> fmt, Args &&...args)
+inline void trace(format_string_t<Args...> fmt, Args &&...args)
 {
     default_logger_raw()->trace(fmt, std::forward<Args>(args)...);
 }
 
 template<typename... Args>
-inline void debug(fmt::format_string<Args...> fmt, Args &&...args)
+inline void debug(format_string_t<Args...> fmt, Args &&...args)
 {
     default_logger_raw()->debug(fmt, std::forward<Args>(args)...);
 }
 
 template<typename... Args>
-inline void info(fmt::format_string<Args...> fmt, Args &&...args)
+inline void info(format_string_t<Args...> fmt, Args &&...args)
 {
     default_logger_raw()->info(fmt, std::forward<Args>(args)...);
 }
 
 template<typename... Args>
-inline void warn(fmt::format_string<Args...> fmt, Args &&...args)
+inline void warn(format_string_t<Args...> fmt, Args &&...args)
 {
     default_logger_raw()->warn(fmt, std::forward<Args>(args)...);
 }
 
 template<typename... Args>
-inline void error(fmt::format_string<Args...> fmt, Args &&...args)
+inline void error(format_string_t<Args...> fmt, Args &&...args)
 {
     default_logger_raw()->error(fmt, std::forward<Args>(args)...);
 }
 
 template<typename... Args>
-inline void critical(fmt::format_string<Args...> fmt, Args &&...args)
+inline void critical(format_string_t<Args...> fmt, Args &&...args)
 {
     default_logger_raw()->critical(fmt, std::forward<Args>(args)...);
 }
@@ -189,49 +189,49 @@ inline void log(level::level_enum lvl, const T &msg)
 
 #ifdef SPDLOG_WCHAR_TO_UTF8_SUPPORT
 template<typename... Args>
-inline void log(source_loc source, level::level_enum lvl, fmt::wformat_string<Args...> fmt, Args &&...args)
+inline void log(source_loc source, level::level_enum lvl, wformat_string_t<Args...> fmt, Args &&...args)
 {
     default_logger_raw()->log(source, lvl, fmt, std::forward<Args>(args)...);
 }
 
 template<typename... Args>
-inline void log(level::level_enum lvl, fmt::wformat_string<Args...> fmt, Args &&...args)
+inline void log(level::level_enum lvl, wformat_string_t<Args...> fmt, Args &&...args)
 {
     default_logger_raw()->log(source_loc{}, lvl, fmt, std::forward<Args>(args)...);
 }
 
 template<typename... Args>
-inline void trace(fmt::wformat_string<Args...> fmt, Args &&...args)
+inline void trace(wformat_string_t<Args...> fmt, Args &&...args)
 {
     default_logger_raw()->trace(fmt, std::forward<Args>(args)...);
 }
 
 template<typename... Args>
-inline void debug(fmt::wformat_string<Args...> fmt, Args &&...args)
+inline void debug(wformat_string_t<Args...> fmt, Args &&...args)
 {
     default_logger_raw()->debug(fmt, std::forward<Args>(args)...);
 }
 
 template<typename... Args>
-inline void info(fmt::wformat_string<Args...> fmt, Args &&...args)
+inline void info(wformat_string_t<Args...> fmt, Args &&...args)
 {
     default_logger_raw()->info(fmt, std::forward<Args>(args)...);
 }
 
 template<typename... Args>
-inline void warn(fmt::wformat_string<Args...> fmt, Args &&...args)
+inline void warn(wformat_string_t<Args...> fmt, Args &&...args)
 {
     default_logger_raw()->warn(fmt, std::forward<Args>(args)...);
 }
 
 template<typename... Args>
-inline void error(fmt::wformat_string<Args...> fmt, Args &&...args)
+inline void error(wformat_string_t<Args...> fmt, Args &&...args)
 {
     default_logger_raw()->error(fmt, std::forward<Args>(args)...);
 }
 
 template<typename... Args>
-inline void critical(fmt::wformat_string<Args...> fmt, Args &&...args)
+inline void critical(wformat_string_t<Args...> fmt, Args &&...args)
 {
     default_logger_raw()->critical(fmt, std::forward<Args>(args)...);
 }
diff --git a/include/spdlog/tweakme.h b/include/spdlog/tweakme.h
index 24361f30..08f0f4e5 100644
--- a/include/spdlog/tweakme.h
+++ b/include/spdlog/tweakme.h
@@ -74,6 +74,13 @@
 // #define SPDLOG_FMT_EXTERNAL
 ///////////////////////////////////////////////////////////////////////////////
 
+///////////////////////////////////////////////////////////////////////////////
+// Uncomment to use C++20 std::format instead of fmt. This removes compile
+// time checking of format strings, but doesn't depend on the fmt library.
+//
+// #define SPDLOG_USE_STD_FORMAT
+///////////////////////////////////////////////////////////////////////////////
+
 ///////////////////////////////////////////////////////////////////////////////
 // Uncomment to enable wchar_t support (convert to utf8)
 //
diff --git a/src/fmt.cpp b/src/fmt.cpp
index b3cd3cf1..536aa675 100644
--- a/src/fmt.cpp
+++ b/src/fmt.cpp
@@ -6,7 +6,7 @@
 #    error Please define SPDLOG_COMPILED_LIB to compile this file.
 #endif
 
-#if !defined(SPDLOG_FMT_EXTERNAL)
+#if !defined(SPDLOG_FMT_EXTERNAL) && !defined(SPDLOG_USE_STD_FORMAT)
 #    include <spdlog/fmt/bundled/format-inl.h>
 
 FMT_BEGIN_NAMESPACE
diff --git a/tests/test_async.cpp b/tests/test_async.cpp
index 83fc0ec6..124f3132 100644
--- a/tests/test_async.cpp
+++ b/tests/test_async.cpp
@@ -166,7 +166,7 @@ TEST_CASE("to_file", "[async]")
     require_message_count(TEST_FILENAME, messages);
     auto contents = file_contents(TEST_FILENAME);
     using spdlog::details::os::default_eol;
-    REQUIRE(ends_with(contents, fmt::format("Hello message #1023{}", default_eol)));
+    REQUIRE(ends_with(contents, spdlog::fmt_lib::format("Hello message #1023{}", default_eol)));
 }
 
 TEST_CASE("to_file multi-workers", "[async]")
diff --git a/tests/test_daily_logger.cpp b/tests/test_daily_logger.cpp
index 6aeff5b1..93bd4662 100644
--- a/tests/test_daily_logger.cpp
+++ b/tests/test_daily_logger.cpp
@@ -3,7 +3,11 @@
  */
 #include "includes.h"
 
+#ifdef SPDLOG_USE_STD_FORMAT
+using filename_memory_buf_t = std::basic_string<spdlog::filename_t::value_type>;
+#else
 using filename_memory_buf_t = fmt::basic_memory_buffer<spdlog::filename_t::value_type, 250>;
+#endif
 
 TEST_CASE("daily_logger with dateonly calculator", "[daily_logger]")
 {
@@ -15,7 +19,7 @@ TEST_CASE("daily_logger with dateonly calculator", "[daily_logger]")
     spdlog::filename_t basename = SPDLOG_FILENAME_T("test_logs/daily_dateonly");
     std::tm tm = spdlog::details::os::localtime();
     filename_memory_buf_t w;
-    fmt::format_to(
+    spdlog::fmt_lib::format_to(
         std::back_inserter(w), SPDLOG_FILENAME_T("{}_{:04d}-{:02d}-{:02d}"), basename, tm.tm_year + 1900, tm.tm_mon + 1, tm.tm_mday);
 
     auto logger = spdlog::create<sink_type>("logger", basename, 0, 0);
@@ -29,9 +33,17 @@ TEST_CASE("daily_logger with dateonly calculator", "[daily_logger]")
 #ifdef SPDLOG_WCHAR_FILENAMES
     spdlog::memory_buf_t buf;
     spdlog::details::os::wstr_to_utf8buf(fmt::to_string(w), buf);
+#ifdef SPDLOG_USE_STD_FORMAT
+    auto &filename = buf;
+#else
     auto filename = fmt::to_string(buf);
+#endif
+#else
+#ifdef SPDLOG_USE_STD_FORMAT
+    auto &filename = w;
 #else
     auto filename = fmt::to_string(w);
+#endif
 #endif
     require_message_count(filename, 10);
 }
@@ -41,9 +53,13 @@ struct custom_daily_file_name_calculator
     static spdlog::filename_t calc_filename(const spdlog::filename_t &basename, const tm &now_tm)
     {
         filename_memory_buf_t w;
-        fmt::format_to(std::back_inserter(w), SPDLOG_FILENAME_T("{}{:04d}{:02d}{:02d}"), basename, now_tm.tm_year + 1900, now_tm.tm_mon + 1,
+        spdlog::fmt_lib::format_to(std::back_inserter(w), SPDLOG_FILENAME_T("{}{:04d}{:02d}{:02d}"), basename, now_tm.tm_year + 1900, now_tm.tm_mon + 1,
             now_tm.tm_mday);
+#ifdef SPDLOG_USE_STD_FORMAT
+        return w;
+#else
         return fmt::to_string(w);
+#endif
     }
 };
 
@@ -57,7 +73,7 @@ TEST_CASE("daily_logger with custom calculator", "[daily_logger]")
     spdlog::filename_t basename = SPDLOG_FILENAME_T("test_logs/daily_dateonly");
     std::tm tm = spdlog::details::os::localtime();
     filename_memory_buf_t w;
-    fmt::format_to(
+    spdlog::fmt_lib::format_to(
         std::back_inserter(w), SPDLOG_FILENAME_T("{}{:04d}{:02d}{:02d}"), basename, tm.tm_year + 1900, tm.tm_mon + 1, tm.tm_mday);
 
     auto logger = spdlog::create<sink_type>("logger", basename, 0, 0);
@@ -71,9 +87,17 @@ TEST_CASE("daily_logger with custom calculator", "[daily_logger]")
 #ifdef SPDLOG_WCHAR_FILENAMES
     spdlog::memory_buf_t buf;
     spdlog::details::os::wstr_to_utf8buf(fmt::to_string(w), buf);
+#ifdef SPDLOG_USE_STD_FORMAT
+    auto &filename = buf;
+#else
     auto filename = fmt::to_string(buf);
+#endif
+#else
+#ifdef SPDLOG_USE_STD_FORMAT
+    auto &filename = w;
 #else
     auto filename = fmt::to_string(w);
+#endif
 #endif
     require_message_count(filename, 10);
 }
diff --git a/tests/test_errors.cpp b/tests/test_errors.cpp
index bcc21d73..41441d0d 100644
--- a/tests/test_errors.cpp
+++ b/tests/test_errors.cpp
@@ -34,7 +34,7 @@ TEST_CASE("default_error_handler", "[errors]]")
     logger->flush();
 
     using spdlog::details::os::default_eol;
-    REQUIRE(file_contents(SIMPLE_LOG) == fmt::format("Test message 2{}", default_eol));
+    REQUIRE(file_contents(SIMPLE_LOG) == spdlog::fmt_lib::format("Test message 2{}", default_eol));
     REQUIRE(count_lines(SIMPLE_LOG) == 1);
 }
 
diff --git a/tests/test_file_helper.cpp b/tests/test_file_helper.cpp
index 37c929f5..168c6601 100644
--- a/tests/test_file_helper.cpp
+++ b/tests/test_file_helper.cpp
@@ -10,7 +10,7 @@ using spdlog::details::file_helper;
 static void write_with_helper(file_helper &helper, size_t howmany)
 {
     spdlog::memory_buf_t formatted;
-    fmt::format_to(std::back_inserter(formatted), "{}", std::string(howmany, '1'));
+    spdlog::fmt_lib::format_to(std::back_inserter(formatted), "{}", std::string(howmany, '1'));
     helper.write(formatted);
     helper.flush();
 }
diff --git a/tests/test_fmt_helper.cpp b/tests/test_fmt_helper.cpp
index 3ef18abe..6972d2d2 100644
--- a/tests/test_fmt_helper.cpp
+++ b/tests/test_fmt_helper.cpp
@@ -8,28 +8,48 @@ void test_pad2(int n, const char *expected)
 {
     memory_buf_t buf;
     spdlog::details::fmt_helper::pad2(n, buf);
+
+#ifdef SPDLOG_USE_STD_FORMAT
+    REQUIRE(formatted == expected);
+#else
     REQUIRE(fmt::to_string(buf) == expected);
+#endif
 }
 
 void test_pad3(uint32_t n, const char *expected)
 {
     memory_buf_t buf;
     spdlog::details::fmt_helper::pad3(n, buf);
+
+#ifdef SPDLOG_USE_STD_FORMAT
+    REQUIRE(formatted == expected);
+#else
     REQUIRE(fmt::to_string(buf) == expected);
+#endif
 }
 
 void test_pad6(std::size_t n, const char *expected)
 {
     memory_buf_t buf;
     spdlog::details::fmt_helper::pad6(n, buf);
+
+#ifdef SPDLOG_USE_STD_FORMAT
+    REQUIRE(formatted == expected);
+#else
     REQUIRE(fmt::to_string(buf) == expected);
+#endif
 }
 
 void test_pad9(std::size_t n, const char *expected)
 {
     memory_buf_t buf;
     spdlog::details::fmt_helper::pad9(n, buf);
+
+#ifdef SPDLOG_USE_STD_FORMAT
+    REQUIRE(formatted == expected);
+#else
     REQUIRE(fmt::to_string(buf) == expected);
+#endif
 }
 
 TEST_CASE("pad2", "[fmt_helper]")
diff --git a/tests/test_macros.cpp b/tests/test_macros.cpp
index dd21ed19..b4ad5654 100644
--- a/tests/test_macros.cpp
+++ b/tests/test_macros.cpp
@@ -25,7 +25,7 @@ TEST_CASE("debug and trace w/o format string", "[macros]]")
     logger->flush();
 
     using spdlog::details::os::default_eol;
-    REQUIRE(ends_with(file_contents(TEST_FILENAME), fmt::format("Test message 2{}", default_eol)));
+    REQUIRE(ends_with(file_contents(TEST_FILENAME), spdlog::fmt_lib::format("Test message 2{}", default_eol)));
     REQUIRE(count_lines(TEST_FILENAME) == 1);
 
     spdlog::set_default_logger(logger);
@@ -35,7 +35,7 @@ TEST_CASE("debug and trace w/o format string", "[macros]]")
     logger->flush();
 
     require_message_count(TEST_FILENAME, 2);
-    REQUIRE(ends_with(file_contents(TEST_FILENAME), fmt::format("Test message 4{}", default_eol)));
+    REQUIRE(ends_with(file_contents(TEST_FILENAME), spdlog::fmt_lib::format("Test message 4{}", default_eol)));
 }
 
 TEST_CASE("disable param evaluation", "[macros]")
diff --git a/tests/test_pattern_formatter.cpp b/tests/test_pattern_formatter.cpp
index 656f93a0..78c4cd30 100644
--- a/tests/test_pattern_formatter.cpp
+++ b/tests/test_pattern_formatter.cpp
@@ -64,7 +64,7 @@ TEST_CASE("color range test1", "[pattern_formatter]")
     auto formatter = std::make_shared<spdlog::pattern_formatter>("%^%v%$", spdlog::pattern_time_type::local, "\n");
 
     memory_buf_t buf;
-    fmt::format_to(std::back_inserter(buf), "Hello");
+    spdlog::fmt_lib::format_to(std::back_inserter(buf), "Hello");
     memory_buf_t formatted;
     std::string logger_name = "test";
     spdlog::details::log_msg msg(logger_name, spdlog::level::info, spdlog::string_view_t(buf.data(), buf.size()));
@@ -273,7 +273,11 @@ TEST_CASE("clone-default-formatter", "[pattern_formatter]")
     formatter_1->format(msg, formatted_1);
     formatter_2->format(msg, formatted_2);
 
+#ifdef SPDLOG_USE_STD_FORMAT
+    REQUIRE(formatted_1 == formatted_2);
+#else
     REQUIRE(fmt::to_string(formatted_1) == fmt::to_string(formatted_2));
+#endif
 }
 
 TEST_CASE("clone-default-formatter2", "[pattern_formatter]")
@@ -288,7 +292,11 @@ TEST_CASE("clone-default-formatter2", "[pattern_formatter]")
     formatter_1->format(msg, formatted_1);
     formatter_2->format(msg, formatted_2);
 
+#ifdef SPDLOG_USE_STD_FORMAT
+    REQUIRE(formatted_1 == formatted_2);
+#else
     REQUIRE(fmt::to_string(formatted_1) == fmt::to_string(formatted_2));
+#endif
 }
 
 TEST_CASE("clone-formatter", "[pattern_formatter]")
@@ -302,7 +310,12 @@ TEST_CASE("clone-formatter", "[pattern_formatter]")
     memory_buf_t formatted_2;
     formatter_1->format(msg, formatted_1);
     formatter_2->format(msg, formatted_2);
+
+#ifdef SPDLOG_USE_STD_FORMAT
+    REQUIRE(formatted_1 == formatted_2);
+#else
     REQUIRE(fmt::to_string(formatted_1) == fmt::to_string(formatted_2));
+#endif
 }
 
 TEST_CASE("clone-formatter-2", "[pattern_formatter]")
@@ -317,7 +330,12 @@ TEST_CASE("clone-formatter-2", "[pattern_formatter]")
     memory_buf_t formatted_2;
     formatter_1->format(msg, formatted_1);
     formatter_2->format(msg, formatted_2);
+
+#ifdef SPDLOG_USE_STD_FORMAT
+    REQUIRE(formatted_1 == formatted_2);
+#else
     REQUIRE(fmt::to_string(formatted_1) == fmt::to_string(formatted_2));
+#endif
 }
 
 class custom_test_flag : public spdlog::custom_flag_formatter
@@ -362,9 +380,15 @@ TEST_CASE("clone-custom_formatter", "[pattern_formatter]")
     formatter_1->format(msg, formatted_1);
     formatter_2->format(msg, formatted_2);
 
-    auto expected = fmt::format("[logger-name] [custom_output] some message{}", spdlog::details::os::default_eol);
+    auto expected = spdlog::fmt_lib::format("[logger-name] [custom_output] some message{}", spdlog::details::os::default_eol);
+
+#ifdef SPDLOG_USE_STD_FORMAT
+    REQUIRE(formatted_1 == expected);
+    REQUIRE(formatted_2 == expected);
+#else
     REQUIRE(fmt::to_string(formatted_1) == expected);
     REQUIRE(fmt::to_string(formatted_2) == expected);
+#endif
 }
 
 //
@@ -385,7 +409,12 @@ TEST_CASE("short filename formatter-1", "[pattern_formatter]")
     spdlog::source_loc source_loc{test_path, 123, "some_func()"};
     spdlog::details::log_msg msg(source_loc, "logger-name", spdlog::level::info, "Hello");
     formatter.format(msg, formatted);
+
+#ifdef SPDLOG_USE_STD_FORMAT
+    REQUIRE(formatted == "myfile.cpp");
+#else
     REQUIRE(fmt::to_string(formatted) == "myfile.cpp");
+#endif
 }
 
 TEST_CASE("short filename formatter-2", "[pattern_formatter]")
@@ -396,7 +425,12 @@ TEST_CASE("short filename formatter-2", "[pattern_formatter]")
     spdlog::source_loc source_loc{"myfile.cpp", 123, "some_func()"};
     spdlog::details::log_msg msg(source_loc, "logger-name", spdlog::level::info, "Hello");
     formatter.format(msg, formatted);
+
+#ifdef SPDLOG_USE_STD_FORMAT
+    REQUIRE(formatted == "myfile.cpp:123");
+#else
     REQUIRE(fmt::to_string(formatted) == "myfile.cpp:123");
+#endif
 }
 
 TEST_CASE("short filename formatter-3", "[pattern_formatter]")
@@ -407,7 +441,12 @@ TEST_CASE("short filename formatter-3", "[pattern_formatter]")
     spdlog::source_loc source_loc{"", 123, "some_func()"};
     spdlog::details::log_msg msg(source_loc, "logger-name", spdlog::level::info, "Hello");
     formatter.format(msg, formatted);
+
+#ifdef SPDLOG_USE_STD_FORMAT
+    REQUIRE(formatted == " Hello");
+#else
     REQUIRE(fmt::to_string(formatted) == " Hello");
+#endif
 }
 
 TEST_CASE("full filename formatter", "[pattern_formatter]")
@@ -418,7 +457,12 @@ TEST_CASE("full filename formatter", "[pattern_formatter]")
     spdlog::source_loc source_loc{test_path, 123, "some_func()"};
     spdlog::details::log_msg msg(source_loc, "logger-name", spdlog::level::info, "Hello");
     formatter.format(msg, formatted);
+
+#ifdef SPDLOG_USE_STD_FORMAT
+    REQUIRE(formatted == test_path);
+#else
     REQUIRE(fmt::to_string(formatted) == test_path);
+#endif
 }
 
 TEST_CASE("custom flags", "[pattern_formatter]")
@@ -430,8 +474,13 @@ TEST_CASE("custom flags", "[pattern_formatter]")
 
     spdlog::details::log_msg msg(spdlog::source_loc{}, "logger-name", spdlog::level::info, "some message");
     formatter->format(msg, formatted);
-    auto expected = fmt::format("[logger-name] [custom1] [custom2] some message{}", spdlog::details::os::default_eol);
+    auto expected = spdlog::fmt_lib::format("[logger-name] [custom1] [custom2] some message{}", spdlog::details::os::default_eol);
+
+#ifdef SPDLOG_USE_STD_FORMAT
+    REQUIRE(formatted == expected);
+#else
     REQUIRE(fmt::to_string(formatted) == expected);
+#endif
 }
 
 TEST_CASE("custom flags-padding", "[pattern_formatter]")
@@ -443,8 +492,13 @@ TEST_CASE("custom flags-padding", "[pattern_formatter]")
 
     spdlog::details::log_msg msg(spdlog::source_loc{}, "logger-name", spdlog::level::info, "some message");
     formatter->format(msg, formatted);
-    auto expected = fmt::format("[logger-name] [custom1] [     custom2] some message{}", spdlog::details::os::default_eol);
+    auto expected = spdlog::fmt_lib::format("[logger-name] [custom1] [     custom2] some message{}", spdlog::details::os::default_eol);
+
+#ifdef SPDLOG_USE_STD_FORMAT
+    REQUIRE(formatted == expected);
+#else
     REQUIRE(fmt::to_string(formatted) == expected);
+#endif
 }
 
 TEST_CASE("custom flags-exception", "[pattern_formatter]")

From a31ae23db11ce96acec36a493d1f1ee805913f11 Mon Sep 17 00:00:00 2001
From: Charless Milette <charles.milette@gmail.com>
Date: Sat, 13 Nov 2021 11:43:19 -0500
Subject: [PATCH 02/21] Fix build issue when using built-in fmt

---
 include/spdlog/common.h | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/include/spdlog/common.h b/include/spdlog/common.h
index fb45d2a2..76370d4f 100644
--- a/include/spdlog/common.h
+++ b/include/spdlog/common.h
@@ -148,7 +148,7 @@ template<typename... Args>
 using format_string_t = fmt::format_string<Args...>;
 
 template<typename... Args>
-using wformat_string_t = fmt::wformat_string<Args...>;
+using wformat_string_t = fmt::basic_format_string<wchar_t, Args...>;
 
 template<class T>
 using remove_cvref_t = typename std::remove_cv<typename std::remove_reference<T>::type>::type;

From c475418975ebf07ccebed860505f80966c38653e Mon Sep 17 00:00:00 2001
From: Charless Milette <charles.milette@gmail.com>
Date: Sat, 13 Nov 2021 11:50:26 -0500
Subject: [PATCH 03/21] Put formatter specialization in its original namespace

---
 include/spdlog/fmt/bin_to_hex.h | 14 ++++++++++++--
 1 file changed, 12 insertions(+), 2 deletions(-)

diff --git a/include/spdlog/fmt/bin_to_hex.h b/include/spdlog/fmt/bin_to_hex.h
index 62807d63..cf0c84ff 100644
--- a/include/spdlog/fmt/bin_to_hex.h
+++ b/include/spdlog/fmt/bin_to_hex.h
@@ -74,8 +74,18 @@ inline details::dump_info<It> to_hex(const It range_begin, const It range_end, s
     return details::dump_info<It>(range_begin, range_end, size_per_line);
 }
 
+} // namespace spdlog
+
+namespace
+#ifdef SPDLOG_USE_STD_FORMAT
+    std
+#else
+    fmt
+#endif
+{
+
 template<typename T>
-struct fmt_lib::formatter<spdlog::details::dump_info<T>, char>
+struct formatter<spdlog::details::dump_info<T>, char>
 {
     const char delimiter = ' ';
     bool put_newlines = true;
@@ -210,4 +220,4 @@ struct fmt_lib::formatter<spdlog::details::dump_info<T>, char>
         }
     }
 };
-} // namespace spdlog
+} // namespace fmt/std

From 4008f31add3d7719abe1713b5354f31892d73fcd Mon Sep 17 00:00:00 2001
From: Charless Milette <charles.milette@gmail.com>
Date: Sat, 13 Nov 2021 11:51:22 -0500
Subject: [PATCH 04/21] Fix missing spdlog::

---
 include/spdlog/fmt/bin_to_hex.h | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/include/spdlog/fmt/bin_to_hex.h b/include/spdlog/fmt/bin_to_hex.h
index cf0c84ff..ab06651b 100644
--- a/include/spdlog/fmt/bin_to_hex.h
+++ b/include/spdlog/fmt/bin_to_hex.h
@@ -216,7 +216,7 @@ struct formatter<spdlog::details::dump_info<T>, char>
 
         if (put_positions)
         {
-            fmt_lib::format_to(inserter, "{:04X}: ", pos);
+            spdlog::fmt_lib::format_to(inserter, "{:04X}: ", pos);
         }
     }
 };

From 6ff1b83038ebbcbc6b0810b1f4d5daf51174efa7 Mon Sep 17 00:00:00 2001
From: Charless Milette <charles.milette@gmail.com>
Date: Sat, 13 Nov 2021 11:54:06 -0500
Subject: [PATCH 05/21] Fix usage of std::forward

---
 include/spdlog/logger.h | 8 ++++----
 1 file changed, 4 insertions(+), 4 deletions(-)

diff --git a/include/spdlog/logger.h b/include/spdlog/logger.h
index 9adfec97..4ce41b84 100644
--- a/include/spdlog/logger.h
+++ b/include/spdlog/logger.h
@@ -364,10 +364,10 @@ protected:
         SPDLOG_TRY
         {
 #ifdef SPDLOG_USE_STD_FORMAT
-            memory_buf_t buf = std::vformat(fmt, std::make_format_args(std::forward<Args...>(args)...));
+            memory_buf_t buf = std::vformat(fmt, std::make_format_args(std::forward<Args>(args)...));
 #else
             memory_buf_t buf;
-            fmt::detail::vformat_to(buf, fmt, fmt::make_format_args(std::forward<Args...>(args)...));
+            fmt::detail::vformat_to(buf, fmt, fmt::make_format_args(std::forward<Args>(args)...));
 #endif
             details::log_msg log_msg(loc, name_, lvl, string_view_t(buf.data(), buf.size()));
             log_it_(log_msg, log_enabled, traceback_enabled);
@@ -390,10 +390,10 @@ protected:
             // format to wmemory_buffer and convert to utf8
             ;
 #ifdef SPDLOG_USE_STD_FORMAT
-            wmemory_buf_t wbuf = std::vformat(fmt, std::make_wformat_args(std::forward<Args...>(args)...));
+            wmemory_buf_t wbuf = std::vformat(fmt, std::make_wformat_args(std::forward<Args>(args)...));
 #else
             wmemory_buf_t wbuf;
-            fmt::detail::vformat_to(wbuf, fmt, fmt::make_format_args<fmt::wformat_context>(std::forward<Args...>(args)...));
+            fmt::detail::vformat_to(wbuf, fmt, fmt::make_format_args<fmt::wformat_context>(std::forward<Args>(args)...));
 #endif
             memory_buf_t buf;
             details::os::wstr_to_utf8buf(wstring_view_t(wbuf.data(), wbuf.size()), buf);

From 89c4b1aabe7463afc15a874dc9602624b8f221ec Mon Sep 17 00:00:00 2001
From: Charless Milette <charles.milette@gmail.com>
Date: Sat, 13 Nov 2021 12:02:40 -0500
Subject: [PATCH 06/21] Fix build issues under C++11

---
 include/spdlog/common.h             | 6 ++++++
 include/spdlog/details/fmt_helper.h | 2 +-
 include/spdlog/fmt/bin_to_hex.h     | 2 +-
 3 files changed, 8 insertions(+), 2 deletions(-)

diff --git a/include/spdlog/common.h b/include/spdlog/common.h
index 76370d4f..caace136 100644
--- a/include/spdlog/common.h
+++ b/include/spdlog/common.h
@@ -55,9 +55,15 @@
 #if defined(_MSC_VER) && (_MSC_VER < 1900)
 #    define SPDLOG_NOEXCEPT _NOEXCEPT
 #    define SPDLOG_CONSTEXPR
+#    define SPDLOG_CONSTEXPR_FUNC
 #else
 #    define SPDLOG_NOEXCEPT noexcept
 #    define SPDLOG_CONSTEXPR constexpr
+#    if __cplusplus >= 201402L
+#        define SPDLOG_CONSTEXPR_FUNC constexpr
+#    else
+#        define SPDLOG_CONSTEXPR_FUNC
+#    endif
 #endif
 
 #if defined(__GNUC__) || defined(__clang__)
diff --git a/include/spdlog/details/fmt_helper.h b/include/spdlog/details/fmt_helper.h
index 31ed64fd..d0b5f98c 100644
--- a/include/spdlog/details/fmt_helper.h
+++ b/include/spdlog/details/fmt_helper.h
@@ -34,7 +34,7 @@ template<typename T>
 inline void append_int(T n, memory_buf_t &dest)
 {
     // Buffer should be large enough to hold all digits (digits10 + 1) and a sign
-    SPDLOG_CONSTEXPR auto BUF_SIZE = std::numeric_limits<T>::digits10 + 2;
+    SPDLOG_CONSTEXPR const auto BUF_SIZE = std::numeric_limits<T>::digits10 + 2;
     char buf[BUF_SIZE];
 
     auto [ptr, ec] = std::to_chars(buf, buf + BUF_SIZE, n, 10);
diff --git a/include/spdlog/fmt/bin_to_hex.h b/include/spdlog/fmt/bin_to_hex.h
index ab06651b..c6aa59d1 100644
--- a/include/spdlog/fmt/bin_to_hex.h
+++ b/include/spdlog/fmt/bin_to_hex.h
@@ -96,7 +96,7 @@ struct formatter<spdlog::details::dump_info<T>, char>
 
     // parse the format string flags
     template<typename ParseContext>
-    SPDLOG_CONSTEXPR auto parse(ParseContext &ctx) -> decltype(ctx.begin())
+    SPDLOG_CONSTEXPR_FUNC auto parse(ParseContext &ctx) -> decltype(ctx.begin())
     {
         auto it = ctx.begin();
         while (it != ctx.end() && *it != '}')

From 48e35f9c3e13ca0792a4ff7eec86ae502e3b1478 Mon Sep 17 00:00:00 2001
From: Charless Milette <charles.milette@gmail.com>
Date: Sat, 13 Nov 2021 12:08:01 -0500
Subject: [PATCH 07/21] Make clang happy, fix VS 2022 generator name

---
 appveyor.yml                        | 2 +-
 include/spdlog/details/fmt_helper.h | 2 +-
 2 files changed, 2 insertions(+), 2 deletions(-)

diff --git a/appveyor.yml b/appveyor.yml
index 42d928e3..6c5f2103 100644
--- a/appveyor.yml
+++ b/appveyor.yml
@@ -66,7 +66,7 @@ environment:
       BUILD_EXAMPLE: 'OFF'
       USE_STD_FORMAT: 'OFF'
       APPVEYOR_BUILD_WORKER_IMAGE: Visual Studio 2019
-    - GENERATOR: '"Visual Studio 16 2022" -A x64'
+    - GENERATOR: '"Visual Studio 17 2022" -A x64'
       BUILD_TYPE: Release
       BUILD_SHARED: 'ON'
       WCHAR: 'OFF'
diff --git a/include/spdlog/details/fmt_helper.h b/include/spdlog/details/fmt_helper.h
index d0b5f98c..25976f50 100644
--- a/include/spdlog/details/fmt_helper.h
+++ b/include/spdlog/details/fmt_helper.h
@@ -57,7 +57,7 @@ inline void append_int(T n, memory_buf_t &dest)
 #endif
 
 template<typename T>
-SPDLOG_CONSTEXPR unsigned int count_digits_fallback(T n)
+SPDLOG_CONSTEXPR_FUNC unsigned int count_digits_fallback(T n)
 {
     // taken from fmt.
     unsigned int count = 1;

From d75de3d3b25ad53b44002fa7760782e3025513a7 Mon Sep 17 00:00:00 2001
From: Charless Milette <charles.milette@gmail.com>
Date: Sun, 14 Nov 2021 02:33:15 -0500
Subject: [PATCH 08/21] Add SPDLOG_USE_STD_FORMAT to target_compile_definitions

---
 CMakeLists.txt | 3 ++-
 1 file changed, 2 insertions(+), 1 deletion(-)

diff --git a/CMakeLists.txt b/CMakeLists.txt
index 2d9bc279..21577e50 100644
--- a/CMakeLists.txt
+++ b/CMakeLists.txt
@@ -231,7 +231,8 @@ foreach(
     SPDLOG_NO_THREAD_ID
     SPDLOG_NO_TLS
     SPDLOG_NO_ATOMIC_LEVELS
-    SPDLOG_DISABLE_DEFAULT_LOGGER)
+    SPDLOG_DISABLE_DEFAULT_LOGGER
+    SPDLOG_USE_STD_FORMAT)
     if(${SPDLOG_OPTION})
         target_compile_definitions(spdlog PUBLIC ${SPDLOG_OPTION})
         target_compile_definitions(spdlog_header_only INTERFACE ${SPDLOG_OPTION})

From 849e90bd017e4b29d2dc691f6a0f33864cabb628 Mon Sep 17 00:00:00 2001
From: Charless Milette <charles.milette@gmail.com>
Date: Mon, 15 Nov 2021 13:36:29 -0500
Subject: [PATCH 09/21] Use /std:c++latest

---
 appveyor.yml | 12 +++++++++++-
 1 file changed, 11 insertions(+), 1 deletion(-)

diff --git a/appveyor.yml b/appveyor.yml
index 6c5f2103..2c951071 100644
--- a/appveyor.yml
+++ b/appveyor.yml
@@ -9,6 +9,7 @@ environment:
       WCHAR_FILES: 'OFF'
       BUILD_EXAMPLE: 'ON'
       USE_STD_FORMAT: 'OFF'
+      CXX_STANDARD: 11
     - GENERATOR: '"Visual Studio 14 2015"'
       BUILD_TYPE: Release
       BUILD_SHARED: 'OFF'
@@ -16,6 +17,7 @@ environment:
       WCHAR_FILES: 'OFF'
       BUILD_EXAMPLE: 'ON'
       USE_STD_FORMAT: 'OFF'
+      CXX_STANDARD: 11
     - GENERATOR: '"Visual Studio 14 2015 Win64"'
       BUILD_TYPE: Debug
       BUILD_SHARED: 'OFF'
@@ -23,6 +25,7 @@ environment:
       WCHAR_FILES: 'OFF'
       BUILD_EXAMPLE: 'ON'
       USE_STD_FORMAT: 'OFF'
+      CXX_STANDARD: 11
     - GENERATOR: '"Visual Studio 14 2015 Win64"'
       BUILD_TYPE: Release
       BUILD_SHARED: 'OFF'
@@ -30,6 +33,7 @@ environment:
       WCHAR_FILES: 'OFF'
       BUILD_EXAMPLE: 'ON'
       USE_STD_FORMAT: 'OFF'
+      CXX_STANDARD: 11
     - GENERATOR: '"Visual Studio 15 2017 Win64"'
       BUILD_TYPE: Debug
       BUILD_SHARED: 'OFF'
@@ -37,6 +41,7 @@ environment:
       WCHAR_FILES: 'OFF'
       BUILD_EXAMPLE: 'ON'
       USE_STD_FORMAT: 'OFF'
+      CXX_STANDARD: 11
     - GENERATOR: '"Visual Studio 15 2017 Win64"'
       BUILD_TYPE: Release
       BUILD_SHARED: 'OFF'
@@ -44,6 +49,7 @@ environment:
       WCHAR_FILES: 'OFF'
       BUILD_EXAMPLE: 'ON'
       USE_STD_FORMAT: 'OFF'
+      CXX_STANDARD: 11
     - GENERATOR: '"Visual Studio 15 2017 Win64"'
       BUILD_TYPE: Release
       BUILD_SHARED: 'ON'
@@ -51,6 +57,7 @@ environment:
       WCHAR_FILES: 'OFF'
       BUILD_EXAMPLE: 'ON'
       USE_STD_FORMAT: 'OFF'
+      CXX_STANDARD: 11
     - GENERATOR: '"Visual Studio 15 2017 Win64"'
       BUILD_TYPE: Release
       BUILD_SHARED: 'ON'
@@ -58,6 +65,7 @@ environment:
       WCHAR_FILES: 'ON'
       BUILD_EXAMPLE: 'OFF'
       USE_STD_FORMAT: 'OFF'
+      CXX_STANDARD: 11
     - GENERATOR: '"Visual Studio 16 2019" -A x64'
       BUILD_TYPE: Release
       BUILD_SHARED: 'ON'
@@ -65,6 +73,7 @@ environment:
       WCHAR_FILES: 'OFF'
       BUILD_EXAMPLE: 'OFF'
       USE_STD_FORMAT: 'OFF'
+      CXX_STANDARD: 17
       APPVEYOR_BUILD_WORKER_IMAGE: Visual Studio 2019
     - GENERATOR: '"Visual Studio 17 2022" -A x64'
       BUILD_TYPE: Release
@@ -73,6 +82,7 @@ environment:
       WCHAR_FILES: 'OFF'
       BUILD_EXAMPLE: 'OFF'
       USE_STD_FORMAT: 'ON'
+      CXX_STANDARD: 23 # std::format is only available with /std:c++latest at the moment.
       APPVEYOR_BUILD_WORKER_IMAGE: Visual Studio 2022
 build_script:
   - cmd: >-
@@ -84,7 +94,7 @@ build_script:
 
       set PATH=%PATH%;C:\Program Files\Git\usr\bin
 
-      cmake -G %GENERATOR% -D CMAKE_BUILD_TYPE=%BUILD_TYPE% -D BUILD_SHARED_LIBS=%BUILD_SHARED% -D SPDLOG_WCHAR_SUPPORT=%WCHAR% -D SPDLOG_WCHAR_FILENAMES=%WCHAR_FILES% -D SPDLOG_BUILD_EXAMPLE=%BUILD_EXAMPLE% -D SPDLOG_BUILD_EXAMPLE_HO=%BUILD_EXAMPLE% -D SPDLOG_BUILD_TESTS=ON -D SPDLOG_BUILD_TESTS_HO=OFF -D SPDLOG_BUILD_WARNINGS=ON -D SPDLOG_USE_STD_FORMAT=%USE_STD_FORMAT% ..
+      cmake -G %GENERATOR% -D CMAKE_BUILD_TYPE=%BUILD_TYPE% -D BUILD_SHARED_LIBS=%BUILD_SHARED% -D SPDLOG_WCHAR_SUPPORT=%WCHAR% -D SPDLOG_WCHAR_FILENAMES=%WCHAR_FILES% -D SPDLOG_BUILD_EXAMPLE=%BUILD_EXAMPLE% -D SPDLOG_BUILD_EXAMPLE_HO=%BUILD_EXAMPLE% -D SPDLOG_BUILD_TESTS=ON -D SPDLOG_BUILD_TESTS_HO=OFF -D SPDLOG_BUILD_WARNINGS=ON -D SPDLOG_USE_STD_FORMAT=%USE_STD_FORMAT% -D CMAKE_CXX_STANDARD=%CXX_STANDARD% ..
 
       cmake --build . --config %BUILD_TYPE%
 

From f6901606f5a36047036072d0c6c58f0c9d90bb9f Mon Sep 17 00:00:00 2001
From: Charless Milette <charles.milette@gmail.com>
Date: Mon, 15 Nov 2021 14:57:13 -0500
Subject: [PATCH 10/21] Add std::tm formatter, fix spdlog::stopwatch formatter,
 conditionally use fmt::runtime in test_errors

---
 include/spdlog/fmt/chrono.h | 86 ++++++++++++++++++++++++++++++++++++-
 include/spdlog/stopwatch.h  | 11 ++++-
 tests/test_errors.cpp       | 12 ++++++
 3 files changed, 105 insertions(+), 4 deletions(-)

diff --git a/include/spdlog/fmt/chrono.h b/include/spdlog/fmt/chrono.h
index 83fad2ff..17013bc1 100644
--- a/include/spdlog/fmt/chrono.h
+++ b/include/spdlog/fmt/chrono.h
@@ -4,11 +4,93 @@
 //
 
 #pragma once
+
+#if defined(SPDLOG_USE_STD_FORMAT)
+// Add a formatter for std::tm, since std::format only supports std::chrono
+// taken from fmtlib
+#include <cassert>
+#include <ctime>
+#include <format>
+#include <string>
+#include <string_view>
+#include <type_traits>
+#include <spdlog/common.h>
+
+namespace spdlog::details
+{
+    inline size_t strftime(char *str, size_t count, const char *format, const std::tm *time)
+    {
+        // Assign to a pointer to suppress GCCs -Wformat-nonliteral
+        // First assign the nullptr to suppress -Wsuggest-attribute=format
+        std::size_t (*strftime)(char*, std::size_t, const char*, const std::tm*) = nullptr;
+        strftime = std::strftime;
+        return strftime(str, count, format, time);
+    }
+
+    inline size_t strftime(wchar_t *str, size_t count, const wchar_t *format, const std::tm *time)
+    {
+        // See above
+        std::size_t (*wcsftime)(wchar_t*, std::size_t, const wchar_t*, const std::tm*) = nullptr;
+        wcsftime = std::wcsftime;
+        return wcsftime(str, count, format, time);
+    }
+
+    // Casts a nonnegative integer to unsigned.
+    template <typename Int>
+    SPDLOG_CONSTEXPR auto to_unsigned(Int value) -> typename std::make_unsigned<Int>::type
+    {
+        assert(value >= 0, "negative value");
+        return static_cast<typename std::make_unsigned<Int>::type>(value);
+    }
+}
+
+template <typename Char>
+struct std::formatter<std::tm, Char>
+{
+    template <typename ParseContext>
+    SPDLOG_CONSTEXPR_FUNC auto parse(ParseContext &ctx) -> decltype(ctx.begin())
+    {
+        auto it = ctx.begin();
+        if (it != ctx.end() && *it == ':') ++it;
+        auto end = it;
+        while (end != ctx.end() && *end != '}') ++end;
+        specs = {it, spdlog::details::to_unsigned(end - it)};
+        return end;
+    }
+
+    template <typename FormatContext>
+    auto format(const std::tm &tm, FormatContext &ctx) const -> decltype(ctx.out()) {
+        basic_string<Char> tm_format;
+        tm_format.append(specs);
+        // By appending an extra space we can distinguish an empty result that
+        // indicates insufficient buffer size from a guaranteed non-empty result
+        // https://github.com/fmtlib/fmt/issues/2238
+        tm_format.push_back(' ');
+
+        const size_t MIN_SIZE = 10;
+        basic_string<Char> buf;
+        buf.resize(MIN_SIZE);
+        for (;;)
+        {
+            size_t count = spdlog::details::strftime(buf.data(), buf.size(), tm_format.c_str(), &tm);
+            if (count != 0)
+            {
+                buf.resize(count);
+                break;
+            }
+            buf.resize(buf.size() * 2);
+        }
+        // Remove the extra space.
+        return std::copy(buf.begin(), buf.end() - 1, ctx.out());
+    }
+
+    basic_string_view<Char> specs;
+};
+
+#else
 //
 // include bundled or external copy of fmtlib's chrono support
 //
-
-#if !defined(SPDLOG_USE_STD_FORMAT)
 #    if !defined(SPDLOG_FMT_EXTERNAL)
 #        ifdef SPDLOG_HEADER_ONLY
 #            ifndef FMT_HEADER_ONLY
diff --git a/include/spdlog/stopwatch.h b/include/spdlog/stopwatch.h
index bb976b19..52b4b6ad 100644
--- a/include/spdlog/stopwatch.h
+++ b/include/spdlog/stopwatch.h
@@ -48,7 +48,14 @@ public:
 } // namespace spdlog
 
 // Support for fmt formatting  (e.g. "{:012.9}" or just "{}")
-namespace fmt {
+namespace
+#ifdef SPDLOG_USE_STD_FORMAT
+    std
+#else
+    fmt
+#endif
+{
+
 template<>
 struct formatter<spdlog::stopwatch> : formatter<double>
 {
@@ -58,4 +65,4 @@ struct formatter<spdlog::stopwatch> : formatter<double>
         return formatter<double>::format(sw.elapsed().count(), ctx);
     }
 };
-} // namespace fmt
+} // namespace fmt/std
diff --git a/tests/test_errors.cpp b/tests/test_errors.cpp
index 41441d0d..c7aef812 100644
--- a/tests/test_errors.cpp
+++ b/tests/test_errors.cpp
@@ -29,7 +29,11 @@ TEST_CASE("default_error_handler", "[errors]]")
 
     auto logger = spdlog::create<spdlog::sinks::basic_file_sink_mt>("test-error", filename, true);
     logger->set_pattern("%v");
+#ifdef SPDLOG_USE_STD_FORMAT
+    logger->info("Test message {} {}", 1);
+#else
     logger->info(fmt::runtime("Test message {} {}"), 1);
+#endif
     logger->info("Test message {}", 2);
     logger->flush();
 
@@ -49,7 +53,11 @@ TEST_CASE("custom_error_handler", "[errors]]")
     logger->set_error_handler([=](const std::string &) { throw custom_ex(); });
     logger->info("Good message #1");
 
+#ifdef SPDLOG_USE_STD_FORMAT
+    REQUIRE_THROWS_AS(logger->info("Bad format msg {} {}", "xxx"), custom_ex);
+#else
     REQUIRE_THROWS_AS(logger->info(fmt::runtime("Bad format msg {} {}"), "xxx"), custom_ex);
+#endif
     logger->info("Good message #2");
     require_message_count(SIMPLE_LOG, 2);
 }
@@ -88,7 +96,11 @@ TEST_CASE("async_error_handler", "[errors]]")
             ofs << err_msg;
         });
         logger->info("Good message #1");
+#ifdef SPDLOG_USE_STD_FORMAT
+        logger->info("Bad format msg {} {}", "xxx");
+#else
         logger->info(fmt::runtime("Bad format msg {} {}"), "xxx");
+#endif
         logger->info("Good message #2");
         spdlog::drop("logger"); // force logger to drain the queue and shutdown
     }

From 2d77ef92b02242cecdf5f1e633706421bf2c2088 Mon Sep 17 00:00:00 2001
From: Charless Milette <charles.milette@gmail.com>
Date: Mon, 15 Nov 2021 15:27:34 -0500
Subject: [PATCH 11/21] Avoid specializing std::formatter for std::tm (not a
 great idea after all)

---
 include/spdlog/details/fmt_helper.h    | 17 +++++
 include/spdlog/fmt/chrono.h            | 86 +-------------------------
 include/spdlog/sinks/daily_file_sink.h | 59 +++++++++++++++---
 include/spdlog/stopwatch.h             |  2 +-
 tests/test_file_logging.cpp            |  4 +-
 5 files changed, 73 insertions(+), 95 deletions(-)

diff --git a/include/spdlog/details/fmt_helper.h b/include/spdlog/details/fmt_helper.h
index 25976f50..0a8de309 100644
--- a/include/spdlog/details/fmt_helper.h
+++ b/include/spdlog/details/fmt_helper.h
@@ -164,6 +164,23 @@ inline ToDuration time_fraction(log_clock::time_point tp)
     return duration_cast<ToDuration>(duration) - duration_cast<ToDuration>(secs);
 }
 
+inline size_t strftime(char *str, size_t count, const char *format, const std::tm *time)
+{
+    // Assign to a pointer to suppress GCCs -Wformat-nonliteral
+    // First assign the nullptr to suppress -Wsuggest-attribute=format
+    std::size_t (*strftime)(char*, std::size_t, const char*, const std::tm*) = nullptr;
+    strftime = std::strftime;
+    return strftime(str, count, format, time);
+}
+
+inline size_t strftime(wchar_t *str, size_t count, const wchar_t *format, const std::tm *time)
+{
+    // See above
+    std::size_t (*wcsftime)(wchar_t*, std::size_t, const wchar_t*, const std::tm*) = nullptr;
+    wcsftime = std::wcsftime;
+    return wcsftime(str, count, format, time);
+}
+
 } // namespace fmt_helper
 } // namespace details
 } // namespace spdlog
diff --git a/include/spdlog/fmt/chrono.h b/include/spdlog/fmt/chrono.h
index 17013bc1..83fad2ff 100644
--- a/include/spdlog/fmt/chrono.h
+++ b/include/spdlog/fmt/chrono.h
@@ -4,93 +4,11 @@
 //
 
 #pragma once
-
-#if defined(SPDLOG_USE_STD_FORMAT)
-// Add a formatter for std::tm, since std::format only supports std::chrono
-// taken from fmtlib
-#include <cassert>
-#include <ctime>
-#include <format>
-#include <string>
-#include <string_view>
-#include <type_traits>
-#include <spdlog/common.h>
-
-namespace spdlog::details
-{
-    inline size_t strftime(char *str, size_t count, const char *format, const std::tm *time)
-    {
-        // Assign to a pointer to suppress GCCs -Wformat-nonliteral
-        // First assign the nullptr to suppress -Wsuggest-attribute=format
-        std::size_t (*strftime)(char*, std::size_t, const char*, const std::tm*) = nullptr;
-        strftime = std::strftime;
-        return strftime(str, count, format, time);
-    }
-
-    inline size_t strftime(wchar_t *str, size_t count, const wchar_t *format, const std::tm *time)
-    {
-        // See above
-        std::size_t (*wcsftime)(wchar_t*, std::size_t, const wchar_t*, const std::tm*) = nullptr;
-        wcsftime = std::wcsftime;
-        return wcsftime(str, count, format, time);
-    }
-
-    // Casts a nonnegative integer to unsigned.
-    template <typename Int>
-    SPDLOG_CONSTEXPR auto to_unsigned(Int value) -> typename std::make_unsigned<Int>::type
-    {
-        assert(value >= 0, "negative value");
-        return static_cast<typename std::make_unsigned<Int>::type>(value);
-    }
-}
-
-template <typename Char>
-struct std::formatter<std::tm, Char>
-{
-    template <typename ParseContext>
-    SPDLOG_CONSTEXPR_FUNC auto parse(ParseContext &ctx) -> decltype(ctx.begin())
-    {
-        auto it = ctx.begin();
-        if (it != ctx.end() && *it == ':') ++it;
-        auto end = it;
-        while (end != ctx.end() && *end != '}') ++end;
-        specs = {it, spdlog::details::to_unsigned(end - it)};
-        return end;
-    }
-
-    template <typename FormatContext>
-    auto format(const std::tm &tm, FormatContext &ctx) const -> decltype(ctx.out()) {
-        basic_string<Char> tm_format;
-        tm_format.append(specs);
-        // By appending an extra space we can distinguish an empty result that
-        // indicates insufficient buffer size from a guaranteed non-empty result
-        // https://github.com/fmtlib/fmt/issues/2238
-        tm_format.push_back(' ');
-
-        const size_t MIN_SIZE = 10;
-        basic_string<Char> buf;
-        buf.resize(MIN_SIZE);
-        for (;;)
-        {
-            size_t count = spdlog::details::strftime(buf.data(), buf.size(), tm_format.c_str(), &tm);
-            if (count != 0)
-            {
-                buf.resize(count);
-                break;
-            }
-            buf.resize(buf.size() * 2);
-        }
-        // Remove the extra space.
-        return std::copy(buf.begin(), buf.end() - 1, ctx.out());
-    }
-
-    basic_string_view<Char> specs;
-};
-
-#else
 //
 // include bundled or external copy of fmtlib's chrono support
 //
+
+#if !defined(SPDLOG_USE_STD_FORMAT)
 #    if !defined(SPDLOG_FMT_EXTERNAL)
 #        ifdef SPDLOG_HEADER_ONLY
 #            ifndef FMT_HEADER_ONLY
diff --git a/include/spdlog/sinks/daily_file_sink.h b/include/spdlog/sinks/daily_file_sink.h
index e56ab763..0e41612a 100644
--- a/include/spdlog/sinks/daily_file_sink.h
+++ b/include/spdlog/sinks/daily_file_sink.h
@@ -5,9 +5,9 @@
 
 #include <spdlog/common.h>
 #include <spdlog/details/file_helper.h>
+#include <spdlog/details/fmt_helper.h>
 #include <spdlog/details/null_mutex.h>
 #include <spdlog/fmt/fmt.h>
-#include <spdlog/fmt/chrono.h>
 #include <spdlog/sinks/base_sink.h>
 #include <spdlog/details/os.h>
 #include <spdlog/details/circular_q.h>
@@ -48,14 +48,57 @@ struct daily_filename_format_calculator
 {
     static filename_t calc_filename(const filename_t &filename, const tm &now_tm)
     {
-        // generate fmt datetime format string, e.g. {:%Y-%m-%d}.
-        filename_t fmt_filename = fmt_lib::format(SPDLOG_FILENAME_T("{{:{}}}"), filename);
-#if defined(SPDLOG_USE_STD_FORMAT)
-        return std::vformat(fmt_filename, std::make_format_args(now_tm));
-#elif defined(_MSC_VER) && defined(SPDLOG_WCHAR_FILENAMES) // for some reason msvc doesn't allow fmt::runtime(..) with wchar here
-        return fmt::format(fmt_filename, now_tm);
+        // adapted from fmtlib
+#ifdef SPDLOG_USE_STD_FORMAT
+        filename_t tm_format;
+        tm_format.append(filename);
+        // By appending an extra space we can distinguish an empty result that
+        // indicates insufficient buffer size from a guaranteed non-empty result
+        // https://github.com/fmtlib/fmt/issues/2238
+        tm_format.push_back(' ');
+
+        const size_t MIN_SIZE = 10;
+        filename_t buf;
+        buf.resize(MIN_SIZE);
+        for (;;)
+        {
+            size_t count = details::fmt_helper::strftime(buf.data(), buf.size(), tm_format.c_str(), &now_tm);
+            if (count != 0)
+            {
+                // Remove the extra space.
+                buf.resize(count - 1);
+                break;
+            }
+            buf.resize(buf.size() * 2);
+        }
+        
+        return buf;
 #else
-        return fmt::format(SPDLOG_FMT_RUNTIME(fmt_filename), now_tm);
+    fmt::basic_memory_buffer<Char> tm_format;
+    tm_format.append(specs.begin(), specs.end());
+    // By appending an extra space we can distinguish an empty result that
+    // indicates insufficient buffer size from a guaranteed non-empty result
+    // https://github.com/fmtlib/fmt/issues/2238
+    tm_format.push_back(' ');
+    tm_format.push_back('\0');
+
+    fmt::basic_memory_buffer<Char> buf;
+    size_t start = buf.size();
+    for (;;)
+    {
+        size_t size = buf.capacity() - start;
+        size_t count = details::fmt_helper:::strftime(&buf[start], size, &tm_format[0], &tm);
+        if (count != 0)
+        {
+            // Remove the extra space.
+            buf.resize(start + count - 1);
+            break;
+        }
+        const size_t MIN_GROWTH = 10;
+        buf.reserve(buf.capacity() + (size > MIN_GROWTH ? size : MIN_GROWTH));
+    }
+
+    return fmt::to_string(buf);
 #endif
     }
 };
diff --git a/include/spdlog/stopwatch.h b/include/spdlog/stopwatch.h
index 52b4b6ad..a1542b2d 100644
--- a/include/spdlog/stopwatch.h
+++ b/include/spdlog/stopwatch.h
@@ -42,7 +42,7 @@ public:
 
     void reset()
     {
-        start_tp_ = clock ::now();
+        start_tp_ = clock::now();
     }
 };
 } // namespace spdlog
diff --git a/tests/test_file_logging.cpp b/tests/test_file_logging.cpp
index eab524dc..c808916d 100644
--- a/tests/test_file_logging.cpp
+++ b/tests/test_file_logging.cpp
@@ -20,7 +20,7 @@ TEST_CASE("simple_file_logger", "[simple_logger]]")
     logger->flush();
     require_message_count(SIMPLE_LOG, 2);
     using spdlog::details::os::default_eol;
-    REQUIRE(file_contents(SIMPLE_LOG) == fmt::format("Test message 1{}Test message 2{}", default_eol, default_eol));
+    REQUIRE(file_contents(SIMPLE_LOG) == spdlog::fmt_lib::format("Test message 1{}Test message 2{}", default_eol, default_eol));
 }
 
 TEST_CASE("flush_on", "[flush_on]]")
@@ -41,7 +41,7 @@ TEST_CASE("flush_on", "[flush_on]]")
     require_message_count(SIMPLE_LOG, 3);
     using spdlog::details::os::default_eol;
     REQUIRE(file_contents(SIMPLE_LOG) ==
-            fmt::format("Should not be flushed{}Test message 1{}Test message 2{}", default_eol, default_eol, default_eol));
+            spdlog::fmt_lib::format("Should not be flushed{}Test message 1{}Test message 2{}", default_eol, default_eol, default_eol));
 }
 
 TEST_CASE("rotating_file_logger1", "[rotating_logger]]")

From 108c656e66411d199db2a4e74e1fe768e1c96989 Mon Sep 17 00:00:00 2001
From: Charless Milette <charles.milette@gmail.com>
Date: Mon, 15 Nov 2021 15:29:16 -0500
Subject: [PATCH 12/21] Fix copy-paste mistake

---
 include/spdlog/sinks/daily_file_sink.h | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/include/spdlog/sinks/daily_file_sink.h b/include/spdlog/sinks/daily_file_sink.h
index 0e41612a..9e907cd3 100644
--- a/include/spdlog/sinks/daily_file_sink.h
+++ b/include/spdlog/sinks/daily_file_sink.h
@@ -87,7 +87,7 @@ struct daily_filename_format_calculator
     for (;;)
     {
         size_t size = buf.capacity() - start;
-        size_t count = details::fmt_helper:::strftime(&buf[start], size, &tm_format[0], &tm);
+        size_t count = details::fmt_helper::strftime(&buf[start], size, &tm_format[0], &tm);
         if (count != 0)
         {
             // Remove the extra space.

From a6945d046f3f4f53822a5b7cc296a3ad385f11af Mon Sep 17 00:00:00 2001
From: Charless Milette <charles.milette@gmail.com>
Date: Mon, 15 Nov 2021 15:30:30 -0500
Subject: [PATCH 13/21] Fix use of Char

---
 include/spdlog/sinks/daily_file_sink.h | 4 ++--
 1 file changed, 2 insertions(+), 2 deletions(-)

diff --git a/include/spdlog/sinks/daily_file_sink.h b/include/spdlog/sinks/daily_file_sink.h
index 9e907cd3..c1c87607 100644
--- a/include/spdlog/sinks/daily_file_sink.h
+++ b/include/spdlog/sinks/daily_file_sink.h
@@ -74,7 +74,7 @@ struct daily_filename_format_calculator
         
         return buf;
 #else
-    fmt::basic_memory_buffer<Char> tm_format;
+    fmt::basic_memory_buffer<filename_t::value_type> tm_format;
     tm_format.append(specs.begin(), specs.end());
     // By appending an extra space we can distinguish an empty result that
     // indicates insufficient buffer size from a guaranteed non-empty result
@@ -82,7 +82,7 @@ struct daily_filename_format_calculator
     tm_format.push_back(' ');
     tm_format.push_back('\0');
 
-    fmt::basic_memory_buffer<Char> buf;
+    fmt::basic_memory_buffer<filename_t::value_type> buf;
     size_t start = buf.size();
     for (;;)
     {

From ba120e524b5a0e6e772c67c0aba45d493a22f49f Mon Sep 17 00:00:00 2001
From: Charless Milette <charles.milette@gmail.com>
Date: Mon, 15 Nov 2021 15:46:22 -0500
Subject: [PATCH 14/21] Add unit test for daily_filename_format_calculator

---
 include/spdlog/sinks/daily_file_sink.h |  2 +-
 tests/test_daily_logger.cpp            | 10 ++++++++++
 2 files changed, 11 insertions(+), 1 deletion(-)

diff --git a/include/spdlog/sinks/daily_file_sink.h b/include/spdlog/sinks/daily_file_sink.h
index c1c87607..b3323780 100644
--- a/include/spdlog/sinks/daily_file_sink.h
+++ b/include/spdlog/sinks/daily_file_sink.h
@@ -75,7 +75,7 @@ struct daily_filename_format_calculator
         return buf;
 #else
     fmt::basic_memory_buffer<filename_t::value_type> tm_format;
-    tm_format.append(specs.begin(), specs.end());
+    tm_format.append(filename.c_str(), filename.c_str() + filename.size());
     // By appending an extra space we can distinguish an empty result that
     // indicates insufficient buffer size from a guaranteed non-empty result
     // https://github.com/fmtlib/fmt/issues/2238
diff --git a/tests/test_daily_logger.cpp b/tests/test_daily_logger.cpp
index 93bd4662..66f01a46 100644
--- a/tests/test_daily_logger.cpp
+++ b/tests/test_daily_logger.cpp
@@ -142,6 +142,16 @@ TEST_CASE("daily_file_sink::daily_filename_calculator", "[daily_file_sink]]")
 }
 #endif
 
+TEST_CASE("daily_file_sink::daily_filename_format_calculator", "[daily_file_sink]]")
+{
+    std::tm tm = spdlog::details::os::localtime();
+    // example-YYYY-MM-DD.log
+    auto filename =
+        spdlog::sinks::daily_filename_format_calculator::calc_filename(SPDLOG_FILENAME_T("example-%Y-%m-%d.log"), tm);
+
+    REQUIRE(filename == spdlog::fmt_lib::format(SPDLOG_FILENAME_T("example-{:04d}-{:02d}-{:02d}.log"), tm.tm_year + 1900, tm.tm_mon + 1, tm.tm_mday));
+}
+
 /* Test removal of old files */
 static spdlog::details::log_msg create_msg(std::chrono::seconds offset)
 {

From 95aa159bddb8a84c6ad21eefc6ae26232defa719 Mon Sep 17 00:00:00 2001
From: Charless Milette <charles.milette@gmail.com>
Date: Mon, 15 Nov 2021 15:50:16 -0500
Subject: [PATCH 15/21] Fix daily_filename_format_calculator (hopefully)

---
 include/spdlog/sinks/daily_file_sink.h | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/include/spdlog/sinks/daily_file_sink.h b/include/spdlog/sinks/daily_file_sink.h
index b3323780..8cdf35c1 100644
--- a/include/spdlog/sinks/daily_file_sink.h
+++ b/include/spdlog/sinks/daily_file_sink.h
@@ -87,7 +87,7 @@ struct daily_filename_format_calculator
     for (;;)
     {
         size_t size = buf.capacity() - start;
-        size_t count = details::fmt_helper::strftime(&buf[start], size, &tm_format[0], &tm);
+        size_t count = details::fmt_helper::strftime(&buf[start], size, &tm_format[0], &now_tm);
         if (count != 0)
         {
             // Remove the extra space.

From 0ded003703fb848bcffbc51e9f9aad287d347a08 Mon Sep 17 00:00:00 2001
From: Charles Milette <charles.milette@gmail.com>
Date: Mon, 15 Nov 2021 16:52:31 -0500
Subject: [PATCH 16/21] Fix wchar_t overloads and dump_info formatter

---
 include/spdlog/common.h         | 2 +-
 include/spdlog/fmt/bin_to_hex.h | 2 +-
 2 files changed, 2 insertions(+), 2 deletions(-)

diff --git a/include/spdlog/common.h b/include/spdlog/common.h
index 191ed224..b702c485 100644
--- a/include/spdlog/common.h
+++ b/include/spdlog/common.h
@@ -154,7 +154,7 @@ template<typename... Args>
 using format_string_t = fmt::format_string<Args...>;
 
 template<typename... Args>
-using wformat_string_t = fmt::basic_format_string<wchar_t, Args...>;
+using wformat_string_t = fmt::basic_format_string<wchar_t, fmt::type_identity_t<Args>...>;
 
 template<class T>
 using remove_cvref_t = typename std::remove_cv<typename std::remove_reference<T>::type>::type;
diff --git a/include/spdlog/fmt/bin_to_hex.h b/include/spdlog/fmt/bin_to_hex.h
index 0dd16e7a..835e4f07 100644
--- a/include/spdlog/fmt/bin_to_hex.h
+++ b/include/spdlog/fmt/bin_to_hex.h
@@ -137,7 +137,7 @@ struct formatter<spdlog::details::dump_info<T>, char>
         SPDLOG_CONSTEXPR const char *hex_lower = "0123456789abcdef";
         const char *hex_chars = use_uppercase ? hex_upper : hex_lower;
 
-#if FMT_VERSION < 60000
+#if !defined(SPDLOG_USE_STD_FORMAT) && FMT_VERSION < 60000
         auto inserter = ctx.begin();
 #else
         auto inserter = ctx.out();

From 484bf07379f9b65f7b867254bdcc993642b7027f Mon Sep 17 00:00:00 2001
From: Charles Milette <charles.milette@gmail.com>
Date: Mon, 15 Nov 2021 18:34:40 -0500
Subject: [PATCH 17/21] Fix test_fmt_helper

---
 tests/test_fmt_helper.cpp | 8 ++++----
 1 file changed, 4 insertions(+), 4 deletions(-)

diff --git a/tests/test_fmt_helper.cpp b/tests/test_fmt_helper.cpp
index 6972d2d2..dde5d482 100644
--- a/tests/test_fmt_helper.cpp
+++ b/tests/test_fmt_helper.cpp
@@ -10,7 +10,7 @@ void test_pad2(int n, const char *expected)
     spdlog::details::fmt_helper::pad2(n, buf);
 
 #ifdef SPDLOG_USE_STD_FORMAT
-    REQUIRE(formatted == expected);
+    REQUIRE(buf == expected);
 #else
     REQUIRE(fmt::to_string(buf) == expected);
 #endif
@@ -22,7 +22,7 @@ void test_pad3(uint32_t n, const char *expected)
     spdlog::details::fmt_helper::pad3(n, buf);
 
 #ifdef SPDLOG_USE_STD_FORMAT
-    REQUIRE(formatted == expected);
+    REQUIRE(buf == expected);
 #else
     REQUIRE(fmt::to_string(buf) == expected);
 #endif
@@ -34,7 +34,7 @@ void test_pad6(std::size_t n, const char *expected)
     spdlog::details::fmt_helper::pad6(n, buf);
 
 #ifdef SPDLOG_USE_STD_FORMAT
-    REQUIRE(formatted == expected);
+    REQUIRE(buf == expected);
 #else
     REQUIRE(fmt::to_string(buf) == expected);
 #endif
@@ -46,7 +46,7 @@ void test_pad9(std::size_t n, const char *expected)
     spdlog::details::fmt_helper::pad9(n, buf);
 
 #ifdef SPDLOG_USE_STD_FORMAT
-    REQUIRE(formatted == expected);
+    REQUIRE(buf == expected);
 #else
     REQUIRE(fmt::to_string(buf) == expected);
 #endif

From 5d6af189f13678ae1becad9b1438455a25256102 Mon Sep 17 00:00:00 2001
From: Charles Milette <charles.milette@gmail.com>
Date: Tue, 16 Nov 2021 09:59:48 -0500
Subject: [PATCH 18/21] Use target.capacity() even with std::string

---
 include/spdlog/details/os-inl.h | 8 --------
 1 file changed, 8 deletions(-)

diff --git a/include/spdlog/details/os-inl.h b/include/spdlog/details/os-inl.h
index 9b2ef0d8..e4cddd41 100644
--- a/include/spdlog/details/os-inl.h
+++ b/include/spdlog/details/os-inl.h
@@ -462,12 +462,8 @@ SPDLOG_INLINE void wstr_to_utf8buf(wstring_view_t wstr, memory_buf_t &target)
         return;
     }
 
-#ifdef SPDLOG_USE_STD_FORMAT
-    int result_size = 0;
-#else
     int result_size = static_cast<int>(target.capacity());
     if ((wstr_size + 1) * 2 > result_size)
-#endif
     {
         result_size = ::WideCharToMultiByte(CP_UTF8, 0, wstr.data(), wstr_size, NULL, 0, NULL, NULL);
     }
@@ -501,12 +497,8 @@ SPDLOG_INLINE void utf8_to_wstrbuf(string_view_t str, wmemory_buf_t &target)
         return;
     }
 
-#ifdef SPDLOG_USE_STD_FORMAT
-    int result_size = 0;
-#else
     int result_size = static_cast<int>(target.capacity());
     if (str_size + 1 > result_size)
-#endif
     {
         result_size = ::MultiByteToWideChar(CP_UTF8, MB_ERR_INVALID_CHARS, str.data(), str_size, NULL, 0);
     }

From 701ef172272eca44e95a71724ac18cd61b5d2316 Mon Sep 17 00:00:00 2001
From: Charles Milette <charles.milette@gmail.com>
Date: Tue, 16 Nov 2021 10:05:35 -0500
Subject: [PATCH 19/21] Move strftime to daily_filename_format_calculator

---
 include/spdlog/details/fmt_helper.h    | 17 -----------------
 include/spdlog/sinks/daily_file_sink.h | 24 ++++++++++++++++++++++--
 2 files changed, 22 insertions(+), 19 deletions(-)

diff --git a/include/spdlog/details/fmt_helper.h b/include/spdlog/details/fmt_helper.h
index 0a8de309..25976f50 100644
--- a/include/spdlog/details/fmt_helper.h
+++ b/include/spdlog/details/fmt_helper.h
@@ -164,23 +164,6 @@ inline ToDuration time_fraction(log_clock::time_point tp)
     return duration_cast<ToDuration>(duration) - duration_cast<ToDuration>(secs);
 }
 
-inline size_t strftime(char *str, size_t count, const char *format, const std::tm *time)
-{
-    // Assign to a pointer to suppress GCCs -Wformat-nonliteral
-    // First assign the nullptr to suppress -Wsuggest-attribute=format
-    std::size_t (*strftime)(char*, std::size_t, const char*, const std::tm*) = nullptr;
-    strftime = std::strftime;
-    return strftime(str, count, format, time);
-}
-
-inline size_t strftime(wchar_t *str, size_t count, const wchar_t *format, const std::tm *time)
-{
-    // See above
-    std::size_t (*wcsftime)(wchar_t*, std::size_t, const wchar_t*, const std::tm*) = nullptr;
-    wcsftime = std::wcsftime;
-    return wcsftime(str, count, format, time);
-}
-
 } // namespace fmt_helper
 } // namespace details
 } // namespace spdlog
diff --git a/include/spdlog/sinks/daily_file_sink.h b/include/spdlog/sinks/daily_file_sink.h
index 8cdf35c1..447f2529 100644
--- a/include/spdlog/sinks/daily_file_sink.h
+++ b/include/spdlog/sinks/daily_file_sink.h
@@ -62,7 +62,7 @@ struct daily_filename_format_calculator
         buf.resize(MIN_SIZE);
         for (;;)
         {
-            size_t count = details::fmt_helper::strftime(buf.data(), buf.size(), tm_format.c_str(), &now_tm);
+            size_t count = strftime(buf.data(), buf.size(), tm_format.c_str(), &now_tm);
             if (count != 0)
             {
                 // Remove the extra space.
@@ -87,7 +87,7 @@ struct daily_filename_format_calculator
     for (;;)
     {
         size_t size = buf.capacity() - start;
-        size_t count = details::fmt_helper::strftime(&buf[start], size, &tm_format[0], &now_tm);
+        size_t count = strftime(&buf[start], size, &tm_format[0], &now_tm);
         if (count != 0)
         {
             // Remove the extra space.
@@ -101,6 +101,26 @@ struct daily_filename_format_calculator
     return fmt::to_string(buf);
 #endif
     }
+
+private:
+#if defined __GNUC__
+#    pragma GCC diagnostic push
+#    pragma GCC diagnostic ignored "-Wformat-nonliteral"
+#endif
+
+    static size_t strftime(char *str, size_t count, const char *format, const std::tm *time)
+    {
+        return std::strftime(str, count, format, time);
+    }
+
+    static size_t strftime(wchar_t *str, size_t count, const wchar_t *format, const std::tm *time)
+    {
+        return std::wcsftime(str, count, format, time);
+    }
+
+#if defined(__GNUC__)
+#    pragma GCC diagnostic pop
+#endif
 };
 
 /*

From ad779e486557008158e900f1e0eef170bbf6b3ea Mon Sep 17 00:00:00 2001
From: Charles Milette <charles.milette@gmail.com>
Date: Tue, 16 Nov 2021 10:10:02 -0500
Subject: [PATCH 20/21] Attempt to solve ambiguous symbol on older MSVC

---
 include/spdlog/common.h | 26 ++++++++++++++++----------
 1 file changed, 16 insertions(+), 10 deletions(-)

diff --git a/include/spdlog/common.h b/include/spdlog/common.h
index b702c485..0d170412 100644
--- a/include/spdlog/common.h
+++ b/include/spdlog/common.h
@@ -127,35 +127,33 @@ using err_handler = std::function<void(const std::string &err_msg)>;
 namespace fmt_lib = std;
 
 using string_view_t = std::string_view;
-using wstring_view_t = std::wstring_view;
 using memory_buf_t = std::string;
-using wmemory_buf_t = std::wstring;
 
 template<typename... Args>
 using format_string_t = std::string_view;
 
-template<typename... Args>
-using wformat_string_t = std::wstring_view;
-
 template<class T, class Char = char>
 struct is_convertible_to_basic_format_string
     : std::integral_constant<bool,
           std::is_convertible<T, std::basic_string_view<Char>>::value>
 {};
+
+#    if defined(SPDLOG_WCHAR_FILENAMES) || defined(SPDLOG_WCHAR_TO_UTF8_SUPPORT)
+using wstring_view_t = std::wstring_view;
+using memory_buf_t = std::wstring;
+
+template<typename... Args>
+using wformat_string_t = std::wstring_view;
+#    endif
 #else
 namespace fmt_lib = fmt;
 
 using string_view_t = fmt::basic_string_view<char>;
-using wstring_view_t = fmt::basic_string_view<wchar_t>;
 using memory_buf_t = fmt::basic_memory_buffer<char, 250>;
-using wmemory_buf_t = fmt::basic_memory_buffer<wchar_t, 250>;
 
 template<typename... Args>
 using format_string_t = fmt::format_string<Args...>;
 
-template<typename... Args>
-using wformat_string_t = fmt::basic_format_string<wchar_t, fmt::type_identity_t<Args>...>;
-
 template<class T>
 using remove_cvref_t = typename std::remove_cv<typename std::remove_reference<T>::type>::type;
 
@@ -166,6 +164,14 @@ struct is_convertible_to_basic_format_string
     : std::integral_constant<bool,
           std::is_convertible<T, fmt::basic_string_view<Char>>::value || std::is_same<remove_cvref_t<T>, fmt::basic_runtime<Char>>::value>
 {};
+
+#    if defined(SPDLOG_WCHAR_FILENAMES) || defined(SPDLOG_WCHAR_TO_UTF8_SUPPORT)
+using wstring_view_t = fmt::basic_string_view<wchar_t>;
+using wmemory_buf_t = fmt::basic_memory_buffer<wchar_t, 250>;;
+
+template<typename... Args>
+using wformat_string_t = fmt::wformat_string<Args...>;
+#    endif
 #endif
 
 #ifdef SPDLOG_WCHAR_TO_UTF8_SUPPORT

From 4001032858cb0c4a5d59a58f597b6c1da469c5f9 Mon Sep 17 00:00:00 2001
From: Charles Milette <charles.milette@gmail.com>
Date: Tue, 16 Nov 2021 11:22:30 -0500
Subject: [PATCH 21/21] Add attribution, return to previous code for
 daily_filename_format_calculator with fmtlib

---
 include/spdlog/details/fmt_helper.h    |  2 +-
 include/spdlog/sinks/daily_file_sink.h | 37 +++++++-------------------
 2 files changed, 11 insertions(+), 28 deletions(-)

diff --git a/include/spdlog/details/fmt_helper.h b/include/spdlog/details/fmt_helper.h
index 25976f50..cc59dd62 100644
--- a/include/spdlog/details/fmt_helper.h
+++ b/include/spdlog/details/fmt_helper.h
@@ -59,7 +59,7 @@ inline void append_int(T n, memory_buf_t &dest)
 template<typename T>
 SPDLOG_CONSTEXPR_FUNC unsigned int count_digits_fallback(T n)
 {
-    // taken from fmt.
+    // taken from fmt: https://github.com/fmtlib/fmt/blob/8.0.1/include/fmt/format.h#L899-L912
     unsigned int count = 1;
     for (;;)
     {
diff --git a/include/spdlog/sinks/daily_file_sink.h b/include/spdlog/sinks/daily_file_sink.h
index 447f2529..36701e98 100644
--- a/include/spdlog/sinks/daily_file_sink.h
+++ b/include/spdlog/sinks/daily_file_sink.h
@@ -5,9 +5,9 @@
 
 #include <spdlog/common.h>
 #include <spdlog/details/file_helper.h>
-#include <spdlog/details/fmt_helper.h>
 #include <spdlog/details/null_mutex.h>
 #include <spdlog/fmt/fmt.h>
+#include <spdlog/fmt/chrono.h>
 #include <spdlog/sinks/base_sink.h>
 #include <spdlog/details/os.h>
 #include <spdlog/details/circular_q.h>
@@ -48,8 +48,9 @@ struct daily_filename_format_calculator
 {
     static filename_t calc_filename(const filename_t &filename, const tm &now_tm)
     {
-        // adapted from fmtlib
 #ifdef SPDLOG_USE_STD_FORMAT
+        // adapted from fmtlib: https://github.com/fmtlib/fmt/blob/8.0.1/include/fmt/chrono.h#L522-L546
+
         filename_t tm_format;
         tm_format.append(filename);
         // By appending an extra space we can distinguish an empty result that
@@ -74,31 +75,13 @@ struct daily_filename_format_calculator
         
         return buf;
 #else
-    fmt::basic_memory_buffer<filename_t::value_type> tm_format;
-    tm_format.append(filename.c_str(), filename.c_str() + filename.size());
-    // By appending an extra space we can distinguish an empty result that
-    // indicates insufficient buffer size from a guaranteed non-empty result
-    // https://github.com/fmtlib/fmt/issues/2238
-    tm_format.push_back(' ');
-    tm_format.push_back('\0');
-
-    fmt::basic_memory_buffer<filename_t::value_type> buf;
-    size_t start = buf.size();
-    for (;;)
-    {
-        size_t size = buf.capacity() - start;
-        size_t count = strftime(&buf[start], size, &tm_format[0], &now_tm);
-        if (count != 0)
-        {
-            // Remove the extra space.
-            buf.resize(start + count - 1);
-            break;
-        }
-        const size_t MIN_GROWTH = 10;
-        buf.reserve(buf.capacity() + (size > MIN_GROWTH ? size : MIN_GROWTH));
-    }
-
-    return fmt::to_string(buf);
+        // generate fmt datetime format string, e.g. {:%Y-%m-%d}.
+        filename_t fmt_filename = fmt::format(SPDLOG_FILENAME_T("{{:{}}}"), filename);
+#    if defined(_MSC_VER) && defined(SPDLOG_WCHAR_FILENAMES) // for some reason msvc doesn't allow fmt::runtime(..) with wchar here
+        return fmt::format(fmt_filename, now_tm);
+#    else
+        return fmt::format(SPDLOG_FMT_RUNTIME(fmt_filename), now_tm);
+#    endif
 #endif
     }