https://gcc.gnu.org/bugzilla/show_bug.cgi?id=78940
--- Comment #2 from Jonathan Wakely <redi at gcc dot gnu.org> --- (In reply to Nadav Har'El from comment #1) > I don't know why the standard decided that the default constructor should > not be marked "constexpr", unlike the value-taking constructors. Because the standard requires that the default constructor doesn't initialize the value, for compatibility with "atomic_int i;" in C which leaves it uninitialized. A constexpr constructor must not leave data uninitialized, so they can't be constexpr if they're C-compatible.
