[Bug c++/113008] New: Trivially default constructible requires default member initializer before the end of its enclosing class

[Bug c++/113008] New: Trivially default constructible requires default member initializer before the end of its enclosing class

[barry.revzin at gmail dot com]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=113008

            Bug ID: 113008
           Summary: Trivially default constructible requires default
                    member initializer before the end of its enclosing
                    class
           Product: gcc
           Version: 13.1.0
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: c++
          Assignee: unassigned at gcc dot gnu.org
          Reporter: barry.revzin at gmail dot com
  Target Milestone: ---

This is related to bugs PR96645 and PR88165.

Consider this:

#include <type_traits>

template <typename T, int N>
struct Array {
    T elems[N];
};

struct A {
    struct B {
        int i = 0;
    };

    static constexpr bool v1 = std::is_trivially_default_constructible_v<B>;
    static constexpr bool v2 =
std::is_trivially_default_constructible_v<Array<B, 10>>;
};



Declaring v1 works (and it correctly evaluates to false).

Declaring v2 fails to compile entirely:

opt/compiler-explorer/gcc-trunk-20231213/include/c++/14.0.0/type_traits:3284:7:
  required from 'constexpr const bool
std::is_trivially_default_constructible_v<Array<A::B, 10> >'
 3284 |     = __is_trivially_constructible(_Tp);
      |       ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
<source>:14:37:   required from here
   14 |     static constexpr bool v2 =
std::is_trivially_default_constructible_v<Array<B, 10>>;
      |                                    
^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
/opt/compiler-explorer/gcc-trunk-20231213/include/c++/14.0.0/type_traits:3284:7:
error: default member initializer for 'A::B::i' required before the end of its
enclosing class
 3284 |     = __is_trivially_constructible(_Tp);
      |       ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
<source>:10:15: note: defined here
   10 |         int i = 0;
      |               ^~~~

If we know that B isn't trivially default constructible (A::v1 is initialized
to false), then can't we likewise also know that Array<B, 10> isn't trivially
default constructible?