The first sentence says:

If a template-parameter of a class template, variable template, or alias
template has a default template-argument, each subsequent
template-parameter shall either have a default template-argument
supplied or be a template parameter pack.

However, we were only testing for "not a function function template",
and referring to an older version of the standard.  As far as I can
tell, CWG2032 added the variable-template, and the alias-template
pre-dates the standard on github.

This patch started as a bug fix for llvm#83461 , but ended up fixing a
number of similar cases, so those are validated as well.