(Template) friend class cannot inherit from private member of class declaring friendship

["Kilian, Jens" <jens.kilian@xxxxxxxxxxxxx>]

Is this legal C++?  It's rejected by GCC, but accepted by Clang.

https://godbolt.org/z/GrGk5Y

This version, with a non-template derived class, is accepted:

https://godbolt.org/z/PEAGNS