[libc++] Instantiation of `pair` in class template argument deduction is a bit too eager

[frederick-vs-ja]

The following code snippet is rejected with libc++ (Godbolt link):

#include <utility>
#include <type_traits>

// note that *+[]{} dereferences a function pointer converted from the lambda expression
static_assert(std::is_same_v<decltype(std::pair{0, *+[]{}}), std::pair<int, void(*)()>>);

It seems that the related implicit deduction guide synthesized from the constructor (pair(const T1&, const T2&) -> pair<T1, T2>) should be excluded by SFINAE mechanisms before instantiating invalid pair<int, void()>, and the decaying explicit deduction guide (pair(T1, T2) -> pair<T1, T2>) should work.

But libc++ currently makes the instantiation a bit too eager, and then causes hard error. I've roughly verified that this can (and perhaps should) be fixed within the library implementation.