P2255R2 Type Traits To Detect References Binding To Temporaries

[StephanTLavavej]

P2255R2 Type Traits To Detect References Binding To Temporaries
LWG-3819 reference_meows_from_temporary should not use is_meowible
LWG-4045 tuple can create dangling references from tuple-like
LWG-4148 unique_ptr::operator* should not allow dangling references

Feature-test macro:

#define __cpp_lib_reference_from_temporary 202202L

[StephanTLavavej]

I've contacted the compiler team about getting support for this.

[StephanTLavavej]

• __reference_constructs_from_temporary has been implemented in Clang 18.
• __reference_converts_from_temporary is being implemented in Clang.

[cpplearner]

__reference_converts_from_temporary has been implemented in Clang 19 (llvm/llvm-project@2dbe89d).

[frederick-vs-ja]

It's annoying that the number of constructors of tuple is quite large. Also, it's unclear to me how should the allocator-extended constructors behave for tuples containing references, as uses-allocator construction is only defined for objects.

[StephanTLavavej]

We've picked up Clang 19 in #5247, so I believe this is now unblocked.

[frederick-vs-ja]

In LLVM-118362, it's observed that C++26 core language change WG21-P2748R5 can automatically implement LWG-4148. However, I guess we should implement LWG-4148 in C++23 mode.

[TymianekPL]

I'll start working on it soon enough, I hope to implement it by the end of May or June.

[frederick-vs-ja]

I'm working on this (https://github.com/frederick-vs-ja/STL/tree/p2255r2-clang).

The changes in product code seem to be complete - although they're a bit too complicated for tuple and pair. Test coverage is incomplete.