Clang template incomplete type

Question

I have the following code that compiles fine under in Visual Studio and g++ but in Clang I get the error \"error: \'ns::B\' is an incomplete type\"

A.h

Answer 1

The program is ill-formed, no diagnostic required.

[temp.res]/8:

The program is ill-formed, no diagnostic required, if:

• [...]
• a hypothetical instantiation of a template immediately following its definition would be ill-formed due to a construct that does not depend on a template parameter, or
• [...]