https://gcc.gnu.org/bugzilla/show_bug.cgi?id=78022
--- Comment #3 from fiesh at zefix dot tv --- Interestingly, clang-5 no longer compiles this code: <source>:28:15: error: constexpr variable 'b1' must be initialized by a constant expression constexpr B b1{0}; ^~~~~ <source>:28:15: note: constructor inherited from base class 'A<int>' cannot be used in a constant expression; derived class cannot be implicitly initialized <source>:14:16: note: declared here using A<int>::A; ^ 1 error generated. Compiler returned: 1 So gcc, clang <=4 and clang-5 behave pairwise differently. At most one of them can be right, who might it be?