[expr.prim.id.qual]/3:

… If a nested-name-specifier N is declarative and has a simple-template-id with a template argument list A that involves a template parameter, let T be the template nominated by N without A…
(3.2) — Otherwise, N nominates the partial specialization ([temp.spec.partial]) of T whose template argument list is equivalent to A ([temp.over.link]); the program is ill-formed if no such partial specialization exists.

I suspect that such partial specialization shall not only exists, but be reachable.
This issue is blessed by @opensdh