`<type_traits>`: Investigate one `reference_constructs_from_temporary` case

[StephanTLavavej]

After Jonathan Emmett (@joemmett) fixed internal VSO-2742607 "C1XX __reference_constructs_from_temporary mishandles a couple of cases", there's a remaining point of divergence between C1XX and Clang in the test cases added by #5537. Fully reduced:

D:\GitHub\STL\out\x64>type meow.cpp

#include <type_traits>

struct to_int_prvalue_implicitly {
    explicit(false) operator int() const;
};

struct to_int_prvalue_explicitly {
    explicit(true) operator int() const;
};

static_assert(std::reference_constructs_from_temporary_v<const int&, to_int_prvalue_implicitly>);
static_assert(std::reference_constructs_from_temporary_v<const int&, to_int_prvalue_explicitly>);

D:\GitHub\STL\out\x64>cl
Microsoft (R) C/C++ Optimizing Compiler Version 19.51.36223.2 for x64 (PREVIEW)
Copyright (C) Microsoft Corporation.  All rights reserved.

usage: cl [ option... ] filename... [ /link linkoption... ]

D:\GitHub\STL\out\x64>cl /EHsc /nologo /W4 /std:c++latest /diagnostics:caret /c meow.cpp
meow.cpp
meow.cpp(12,20): error C2607: static assertion failed
static_assert(std::reference_constructs_from_temporary_v<const int&, to_int_prvalue_explicitly>);
                   ^

D:\GitHub\STL\out\x64>clang-cl -v
clang version 20.1.8
Target: x86_64-pc-windows-msvc
Thread model: posix
InstalledDir: C:\Program Files\Microsoft Visual Studio\18\Insiders\VC\Tools\Llvm\x64\bin

D:\GitHub\STL\out\x64>clang-cl /EHsc /nologo /W4 /std:c++latest /c meow.cpp

D:\GitHub\STL\out\x64>

I don't want to try to summarize Jonathan's analysis of this case, since I'll probably mangle it. I believe this is related to a Core issue that we didn't get a response about on the CWG reflector, and we haven't yet rounded up a Clang dev to talk to Jonathan and see if we can get some kind of agreement about what the correct answer should be here. So, I'm filing this tracking issue.

[joemmett]

Yes, this is related to CWG 2267.

std::reference_constructs_from_temporary_v<const int&, to_int_prvalue_explicitly>) involves forming const int& t(to_int_prvalue_explicitly{}), which is case 6 from the core issue. Core's resolution was that this should be ill-formed, therefore the trait should evaluate to false.

The actual wording that made this ill-formed got lost in a merge conflict, however. The wording in the core issue strikes out the "or cv2 T2" case from the list of explicit conversion functions. This wording has undergone a lot of churn but core's opinion when I asked in Nov 2025 was that this case is still intended to be ill-formed but they haven't actually updated the wording.

[StephanTLavavej]

Thanks. So we need to get CWG to fix the Standard, Clang and EDG to implement the fixed wording, and then we can update the tests accordingly.