Why is the defaulted constructor not constexpr here?

Question

The following example seems to compile with Clang but not with gcc. Which one is right?

#include <type_traits>

template<typename T>
struct MyType
{
  constexpr MyType () = default;
  //constexpr MyType () {}

  constexpr bool is_int () const
  {
    return std::is_same_v<T, int>;
  }
};

constexpr auto foo ()
{
  MyType<int> retval;
  //MyType<int> retval {};

  return retval;
}

int main ()
{
  static_assert (foo ().is_int ());
}

Also, note that un-commenting any of the two commented lines (and removing the respective line above it) makes the program compile with gcc, too.

If gcc is right here, why doesn't it compile?

Answer 1

The complaint is that you're using an uninitialized variable in a constexpr function. However - that's not the case, i.e. retval is default-initialized. So GCC is wrong, unless the standard has very weird language (which I doubt, but don't know for sure).

@StoryTeller suggests this is related to this bug report.