https://gcc.gnu.org/bugzilla/show_bug.cgi?id=96645
--- Comment #2 from Sergey <dev at hrookami dot icu> ---
(In reply to Jonathan Wakely from comment #1)
> This is not a bug in std::variant:
>
>
> template<bool B>
> struct bool_constant
> {
> static constexpr bool value = B;
> using type = bool_constant;
> };
>
> using true_type = bool_constant<true>;
>
> template<typename T>
> struct is_default_constructible
> : bool_constant<__is_constructible(T)>
> { };
>
> void testVarStruct()
> {
> struct DataWithStruct {
> struct A {
> int number = 5; // compiles, if remove initialization
> };
>
> is_default_constructible<A>::type t = true_type{};
> };
> }
>
> v.C: In function 'void testVarStruct()':
> v.C:22:47: error: could not convert 'true_type{}' from 'bool_constant<true>'
> to 'bool_constant<false>'
> 22 | is_default_constructible<A>::type t = true_type{};
> | ^~~~~~~~~~~
> | |
> | bool_constant<true>
>
>
> This stopped working with r269032 which changed the result of the
> __is_constructible built-in.
>
> Clang also rejects this, for the same reason, but EDG accepts it.
Clang compiles it if use libc++ instead of libstdc++
