[temp.res.general]/6 gives the rules that determine when a template is valid, but it's specified in terms of "valid specializations". That doesn't seem like it can work for templates that don't have specializations, in particular for concepts and alias templates. Clearly the intent here is that the same rule applies everywhere -- any construct appearing as or within a templated entity is required to be valid for at least one substitution -- but it's currently at best unclear that that follows from the wording.

I think we should replace some or most of the mentions of "specializations" and "instantiations" in that paragraph with words that talk about substitutions being performed into the template. (Also, this paragraph means to talk about templated entities, not just templates.)

It seems sufficiently clear what this wording means to say that I think we can do this editorially, but it probably makes sense to run the result past CWG.