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Update compatibility.md (#1059)

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Add a note about Lua 5.3 change to semantics of __eq metamethod.

We currently do not plan to implement this as this breaks backwards
compatibility and makes these operators harder to reason about from the
type checking perspective.

Fixes #1051.
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Arseny Kapoulkine 2023-10-03 07:39:48 -07:00 committed by GitHub
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docs/_pages/compatibility.md
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@ -57,7 +57,7 @@ Sandboxing challenges are [covered in the dedicated section](sandbox).
| tables honor the `__len` metamethod | ✔️ | |
| hex and `\z` escapes in strings | ✔️ | |
| support for hexadecimal floats | 🤷‍♀️ | no strong use cases |
| order metamethods work for different types | ❌ | no strong use cases and more complicated semantics, compatibility and performance implications |
| order metamethods (`__lt`/`__le`) are called for unrelated metatables | ❌ | no strong use cases and more complicated semantics, compatibility and performance implications |
| empty statement | 🤷‍♀️ | less useful in Lua than in JS/C#/C/C++ |
| `break` statement may appear in the middle of a block | 🤷‍♀️ | we'd like to do it consistently for `break`/`return`/`continue` but there be dragons |
| arguments for function called through `xpcall` | ✔️ | |
@ -92,7 +92,8 @@ Ephemeron tables may be implemented at some point since they do have valid uses
| new function `table.move` | ✔️ | |
| `collectgarbage("count")` now returns only one result | ✔️ | |
| `coroutine.isyieldable` | ✔️ | |
| stricter error checking for `table.insert`/`table.remove` | 😞 | we love this, but it breaks compatibility
| stricter error checking for `table.insert`/`table.remove` | 😞 | we love this, but it breaks compatibility |
| `__eq` metamethod is called for unrelated metatables | ❌ | backwards compatibility and typechecking implications |
It's important to highlight integer support and bitwise operators. For Luau, it's rare that a full 64-bit integer type is necessary - double-precision types support integers up to 2^53 (in Lua which is used in embedded space, integers may be more appealing in environments without a native 64-bit FPU). However, there's a *lot* of value in having a single number type, both from performance perspective and for consistency. Notably, Lua doesn't handle integer overflow properly, so using integers also carries compatibility implications.