CWG2971 [module.global.frag] Specializations for a class are not decl-reachable via it

[opensdh]

Reference (section label): [module.global.frag]

Link to reflector thread (if any): https://anarthal.github.io/cppblog/modules2

Issue description:

/3.5.7 means that X<0> but not Y<Z> is reachable in importers of the following:

module;
// included...
template<int> struct X {};
template<> struct X<0> {};
struct Z;
template<class> struct Y {};
template<> struct Y<Z> {};
// ...done
export module M;
export using ::X;
export using ::Z;

This is not helpful when Y is a trait defined by another library and specialized for the library being exposed as M.

Suggested resolution:

Extend /3.5 to include the case where D is a declaration of a partial or explicit specialization (or explicit instantiation) and M is a declaration of an entity that appears in D's template argument list (recursively).

[jensmaurer]

CWG2971