[temp.deduct.general] p1 just states that：

When a function template specialization is referenced, all of the template arguments shall have values.

So, what are the necessary template arguments when referring to a function template specialization without using a template-id? It seems to we have no special rule that explicitly specify this point. For example:

template<class U, class T>
U fun(T){};
fun(0);  // #1

According to [temp.deduct.call], the deduction is successful for T since the deduction only concerns the function parameter. The note [temp.deduct.call.note]/1 implies that the template-argument must be explicitly specified for U. Besides that, we have no formal rule to interpret why. Although [temp.deduct.general] p1 requires that all of the template arguments shall have values, however in this case, what are the necessary template arguments when referring to the specialization? I would expect that [temp.names] p7 can work here to interpret the aforementioned issue.

A template-id is valid if

• [...]
• there is an argument for each non-deducible non-pack parameter that does not have a default template-argument

Although the specialization referred to at #1 does not syntactically satisfy a template-id, however, it is as if the specialization were referred to by a template-id, merely, the template-argument-list is permitted to omit as per [temp.arg.explicit] p4.

For this issue, we just need a formal rule(like [temp.names] p7) to specify that we need two template arguments for parameters U and T in this case.