https://gcc.gnu.org/bugzilla/show_bug.cgi?id=119486
Bug ID: 119486
Summary: CTAD for std::pair from function lvalue results in
hard error since C++20
Product: gcc
Version: 15.0
Status: UNCONFIRMED
Keywords: rejects-valid
Severity: normal
Priority: P3
Component: libstdc++
Assignee: unassigned at gcc dot gnu.org
Reporter: de34 at live dot cn
Target Milestone: ---
When using GCC w/ libstdc++, the following example is accepted in C++17 but
rejected in C++20.
Curiously, Clang w/ libstdc++ rejects even in C++17.
https://godbolt.org/z/o5KPecMs5
```
#include <type_traits>
#include <utility>
void fun() {}
static_assert(std::is_same_v<decltype(std::pair(fun, fun)),
std::pair<void(*)(), void(*)()>>);
```
It seems to me that the implicit deduction guide should be excluded by SFINAE
before instantiation, while the decaying explicit deduction guide should work.
MSVC STL seems to be correct here.
Related libc++ issue: https://github.com/llvm/llvm-project/issues/133056
