[Bug c++/104512] New: [c++20] consteval constructor does not need to initialize all data members

["fchelnokov at gmail dot com" <gcc-bugzilla@gcc.gnu.org>]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=104512

            Bug ID: 104512
           Summary: [c++20] consteval constructor does not need to
                    initialize all data members
           Product: gcc
           Version: 12.0
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: c++
          Assignee: unassigned at gcc dot gnu.org
          Reporter: fchelnokov at gmail dot com
  Target Milestone: ---

In the next program struct B has immediate consteval default constructor, which
does not initialize i field. Then this constructor is used to make a temporary
and its i field remains untouched:
```
struct B {
    bool b = true;
    int i;
    consteval B() {}
};
static_assert( B{}.b );
```
Clang and MSVC are fine with it. But GCC complains:
 error: 'B{true}' is not a constant expression
    7 | static_assert( B{}.b );
      |                  ^
error: 'B()' is not a constant expression because it refers to an incompletely
initialized variable
Demo: https://gcc.godbolt.org/z/x4n6ezrhT

The requirement that "every non-variant non-static data member must be
initialized" in constexpr/consteval constructor is removed since c++20. Related
discussion: https://stackoverflow.com/a/71085832/7325599