Answer by HolyBlackCat for Why can't C++ concepts use a simple requirement to check if a nested type exists?

I know concept HasInner = requires { typename T::inner; }; is ok. But I want know why the first one is not ok.

Because requires{ T::inner; } expects T::inner to not be a type (but rather a variable or a function). I'm not sure what answer you expected.

It wouldn't be very useful to make T::inner accept both types and non-types.