[Bug c++/77945] New: GCC generates invalid constexpr copy/move assignment operators for types with trailing padding.

["eric at efcs dot ca" <gcc-bugzilla at gcc dot gnu dot org>]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=77945

            Bug ID: 77945
           Summary: GCC generates invalid constexpr copy/move assignment
                    operators for types with trailing padding.
           Product: gcc
           Version: 7.0
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: c++
          Assignee: unassigned at gcc dot gnu.org
          Reporter: eric at efcs dot ca
  Target Milestone: ---

The copy/move assignment operators generated by GCC result in compile time
error when used in a constant expression if the type contains trailing padding
bytes (but one bytes between members).

Reproducer:
// g++ -std=c++1z test.cpp
struct OK {
  char no_padding = 0;
  constexpr OK() {}
};
static_assert((OK{} = OK{}, true), "");

struct T {
  alignas(2) char with_padding = 0;
  constexpr T() {}
};
// emits-error {{non-constant condition in static assert}}
// emits-error {{error accessing value of '<anonymous>' through a 'char [1]'
glvalue in a constant expression}}
static_assert((T{} = T{}, true), "");