template<typename T, typename U = T>
struct Test{};  

template<typename T>
void func(Test<T> /*#1*/){  
}

Consider the above code, at the point of #1, Does the template-id Test<T> require a template argument for corresponding template parameter U? The only quote that mentioned default argument as template argument in the standard is the following :

When a simple-template-id does not name a function, a default template-argument is implicitly instantiated when the value of that default argument is needed. [ Example:
template<typename T, typename U = int> struct S { };
S* p; // the type of p is S<bool, int>*
The default argument for U is instantiated to form the type S<bool, int>*.  — end example ]

However at the point of #1, The specialization of Test<T> is just a part of a function template definition, that is, there's no instantiation occurs here. So, the above quote seems to be not suitable for this case.

In the standard, there's a quote explicitly specified when to supply default arguments as arguments for function call, that is:

If an initializer-clause is specified in a parameter-declaration this initializer-clause is used as a default argument. Default arguments will be used in calls where trailing arguments are missing.

So, Is it necessary to formulate a formally terminology to cover this case? such as

A default template-argument is a template-argument ([temp.arg]) specified after = in a template-parameter. Default arguments will be used in template-id where trailing arguments are missing and the corresponding template parameter does not participate in template argument deduction.