https://gcc.gnu.org/bugzilla/show_bug.cgi?id=87603
--- Comment #6 from Jonathan Wakely <redi at gcc dot gnu.org> ---
Specifically, we get a new FAIL when running the libstdc++ tests in c++2a mode:
FAIL: 21_strings/basic_string/types/1.cc (test for excess errors)
That's because the is_convertible trait instantiates the new c++2a-only
__is_nothrow_type member, which instantiates the basic_string_view<X> ctor,
which is ill-formed because X isn't equality comparable.
I think this might be a rejects-valid example:
template<typename T>
struct basic_string_view
{
constexpr basic_string_view(T p) noexcept { (void) p.i; }
};
struct X { } x;
bool b = noexcept(basic_string_view<X>{x});
ntconv.cc: In instantiation of 'constexpr
basic_string_view<T>::basic_string_view(T) [with T = X]':
ntconv.cc:10:42: required from here
ntconv.cc:5:56: error: 'struct X' has no member named 'i'
5 | constexpr basic_string_view(T p) noexcept { (void) p.i; }
| ~~^
If I'm reading the standard correctly, the constructor's noexcept-specifier is
instantiated because it's "needed" but that should not cause the instantiation
of the function declaration.
If the constructor is not constexpr it isn't instantiated when the
noexcept-specifier is needed, which is why I think it's related to this bug.
