4 ms·

This is absolutely false. https://github.com/llvm-mirror/libcxx/blob/master/include/type_traits https://github.com/llvm-mirror/libcxx/blob/master/include/ty...

• fooker 8y ago
  It checks if a non standard feature is available, otherwise falls back to a standard implementation. My argument was that it was possible to implement it with standard C++.
  • Kranar 8y ago
    There's no way to implement that type trait using standard C++. The implementation does check if a non-standard feature is available, and if not it delegates to is_scalar which in turn delegates to is_enum which in turn delegates to is_union. is_union can not be implemented in a standard conforming manner without compiler support and libc++ unconditionally returns false if compiler support is not available, which does not conform to the standard.