[Bug c++/86966] ICE (Segmentation fault) for an explicit specialization of a member class template

[v.reshetnikov at gmail dot com]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=86966

--- Comment #2 from Vladimir Reshetnikov <v.reshetnikov at gmail dot com> ---
I believe the code is valid. We explicitly specialize the member class template
X of S for S<> (i.e. the parameter pack T is empty). T is expanded into a list
of zero non-type template parameters of S<>::X.

Here is a similar code where we specialize for S<bool>, i.e. provide one
template argument bool for the pack T:

template<class... T>
struct S {
    template<T...>
    struct X { };
};

template<>
template<bool>
struct S<bool>::X { };

Here T is expanded into a list of one non-type template parameter of type bool.
This code compiles successfully.