The short answer is: Clang and GCC are correct. You must use typename A::B

The compilers that accept A::B() or A::B{} without typename are non-compliant in this case.

Why typename is Required Here?

The rule is that you must use the typename keyword to prefix any dependent qualified name that refers to a type.

The entire issue boils down to one question: Is A::B a dependent name? The answer is yes.