[Bug c++/90814] New: gcc accept invalid constexpr defaulted constructor

[tyker at outlook dot com]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=90814

            Bug ID: 90814
           Summary: gcc accept invalid constexpr defaulted constructor
           Product: gcc
           Version: 10.0
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: c++
          Assignee: unassigned at gcc dot gnu.org
          Reporter: tyker at outlook dot com
  Target Milestone: ---

with gcc, the following code doesn't emit an error if INCORRECT is set.

struct A {
  A() {}
};

template<typename T = int>
struct D : A {
#ifdef INCORRECT
  int& r;
#endif
};

struct B {
  D<> d;
  constexpr B() = default;
};

example : https://godbolt.org/z/oAVthP