Clang incorrectly instantiates conditional-explicit even when constraints are not satisfied

[ldionne]

In code like the following, Clang will instantiate the content of the conditional-explicit even when the constraint is not satisfied:

template <some-constraint T>
  explicit(some-expression-involving-T)
constructor(T const&) { }

As a result, if some-expression-involving-T is not well-formed, Clang will issue a hard error even if some-constraint is not satisfied, in which case one would have expected that the constructor is removed from the overload sets instead.

Full reproducer:

// clang++ -xc++ - -std=c++2b -fsyntax-only
#include <type_traits>
#include <utility>

template <class ...T>
struct tuple { };

template <class T>            struct IsPairLikeImpl : std::false_type { };
template <class T1, class T2> struct IsPairLikeImpl<tuple<T1, T2>> : std::true_type { };
template <class T>            concept IsPairLike = IsPairLikeImpl<T>::value;

template <class ...T>
constexpr auto f(tuple<T...> const&) {
    static_assert(sizeof...(T) == 2);
}

template <class T1, class T2>
struct pair {
    template <IsPairLike P>
    explicit(std::is_void_v<decltype(f(std::declval<P>()))>)
    pair(P const&) { }
};

template <class T>
concept can_construct_pair_from = requires (T t) {
    pair<int, int>(t);
};

static_assert(!can_construct_pair_from<tuple<int>>);
static_assert( can_construct_pair_from<tuple<int, int>>);
static_assert(!can_construct_pair_from<tuple<int, int, int>>);

int main() { }

Godbolt link: https://godbolt.org/z/7PjGqdjes

GCC and MSVC accept this code, Clang doesn't. This was discovered while trying to implement pair-like constructors for libc++.

[llvmbot]

@llvm/issue-subscribers-clang-frontend

[llvmbot]

@llvm/issue-subscribers-c-2b

[JMazurkiewicz]

This is possibly duplicate of #59827.

[LYP951018]

It seems that this bug has been fixed by #70548 . The code works with clang trunk.