From 58ef8ef1e754566cb792fef41abb178c7a4b01fc Mon Sep 17 00:00:00 2001
From: karl-police <karl-police2001@bluewin.ch>
Date: Mon, 16 Sep 2024 02:18:35 +0200
Subject: [PATCH] Allow PrimitiveTypeConstraints to unblock on force dispatch
 if they own the type

---
 Analysis/include/Luau/ConstraintSolver.h |  2 +-
 Analysis/src/ConstraintSolver.cpp        | 21 +++++++++++++++++----
 tests/TypeInfer.primitives.test.cpp      | 18 ++++++++++++++++++
 3 files changed, 36 insertions(+), 5 deletions(-)

diff --git a/Analysis/include/Luau/ConstraintSolver.h b/Analysis/include/Luau/ConstraintSolver.h
index c6b4a828..0bb8fdb0 100644
--- a/Analysis/include/Luau/ConstraintSolver.h
+++ b/Analysis/include/Luau/ConstraintSolver.h
@@ -172,7 +172,7 @@ public:
     bool tryDispatch(const TypeAliasExpansionConstraint& c, NotNull<const Constraint> constraint);
     bool tryDispatch(const FunctionCallConstraint& c, NotNull<const Constraint> constraint);
     bool tryDispatch(const FunctionCheckConstraint& c, NotNull<const Constraint> constraint);
-    bool tryDispatch(const PrimitiveTypeConstraint& c, NotNull<const Constraint> constraint);
+    bool tryDispatch(const PrimitiveTypeConstraint& c, NotNull<const Constraint> constraint, bool force);
     bool tryDispatch(const HasPropConstraint& c, NotNull<const Constraint> constraint);
 
 
diff --git a/Analysis/src/ConstraintSolver.cpp b/Analysis/src/ConstraintSolver.cpp
index ae02c60a..83c209bd 100644
--- a/Analysis/src/ConstraintSolver.cpp
+++ b/Analysis/src/ConstraintSolver.cpp
@@ -630,7 +630,7 @@ bool ConstraintSolver::tryDispatch(NotNull<const Constraint> constraint, bool fo
     else if (auto fcc = get<FunctionCheckConstraint>(*constraint))
         success = tryDispatch(*fcc, constraint);
     else if (auto fcc = get<PrimitiveTypeConstraint>(*constraint))
-        success = tryDispatch(*fcc, constraint);
+        success = tryDispatch(*fcc, constraint, force);
     else if (auto hpc = get<HasPropConstraint>(*constraint))
         success = tryDispatch(*hpc, constraint);
     else if (auto spc = get<HasIndexerConstraint>(*constraint))
@@ -1406,7 +1406,7 @@ bool ConstraintSolver::tryDispatch(const FunctionCheckConstraint& c, NotNull<con
     return true;
 }
 
-bool ConstraintSolver::tryDispatch(const PrimitiveTypeConstraint& c, NotNull<const Constraint> constraint)
+bool ConstraintSolver::tryDispatch(const PrimitiveTypeConstraint& c, NotNull<const Constraint> constraint, bool force)
 {
     std::optional<TypeId> expectedType = c.expectedType ? std::make_optional<TypeId>(follow(*c.expectedType)) : std::nullopt;
     if (expectedType && (isBlocked(*expectedType) || get<PendingExpansionType>(*expectedType)))
@@ -1422,8 +1422,21 @@ bool ConstraintSolver::tryDispatch(const PrimitiveTypeConstraint& c, NotNull<con
     // This is probably the only thing that makes this not insane to do.
     if (auto refCount = unresolvedConstraints.find(c.freeType); refCount && *refCount > 1)
     {
-        block(c.freeType, constraint);
-        return false;
+        if (force)
+        {
+            // If we get force dispatched and are the owner of the constraint
+            // Then do not block.
+            if (canMutate(c.freeType, constraint) == false)
+            {
+                block(c.freeType, constraint);
+                return false;
+            }
+        }
+        else
+        {
+            block(c.freeType, constraint);
+            return false;
+        }
     }
 
     TypeId bindTo = c.primitiveType;
diff --git a/tests/TypeInfer.primitives.test.cpp b/tests/TypeInfer.primitives.test.cpp
index c3cce9df..08af221e 100644
--- a/tests/TypeInfer.primitives.test.cpp
+++ b/tests/TypeInfer.primitives.test.cpp
@@ -96,6 +96,24 @@ TEST_CASE_FIXTURE(Fixture, "check_methods_of_number")
     }
 }
 
+TEST_CASE_FIXTURE(BuiltinsFixture, "primitive_types_issue_IfThenElseIf_union_simplification")
+{
+    if (!FFlag::LuauSolverV2)
+        return;
+
+    CheckResult result = check(R"(
+local v1: string?
+
+local stringButItIsHere = "Windows"
+
+v1 = if true then stringButItIsHere
+elseif false then "Something"
+else "Other"
+)");
+
+    LUAU_REQUIRE_NO_ERRORS(result);
+}
+
 TEST_CASE("singleton_types")
 {
     BuiltinsFixture a;