[temp.expl.spec] p10 says

A trailing template-argument can be left unspecified in the template-id naming an explicit function template specialization provided it can be deduced ([temp.deduct.decl]).

This rule cannot expound the intent implied by the example following it. the declarator-id is not a template-id but an identifier. The rule might be changed to

A trailing template-argument can be left unspecified in an explicit specialization of a function template or of a member function template provided it can be deduced ([temp.deduct.decl]).. If all template arguments can be deduced, the empty template argument list <> may be omitted.

In the same manner as [temp.explicit] p8.