[temp.deduct.type] p4 states that:

If a template parameter is used only in non-deduced contexts and is not explicitly specified, template argument deduction fails.

Consider this example:

template<class...T, class U>
void fun(T...,U){
}
fun(0); // all major implementations accept it.

[temp.deduct.type] p5 states that the template parameter pack T is used in a non-deduced context:

The non-deduced contexts are:

• [...]
• A function parameter pack that does not occur at the end of the parameter-declaration-list.

The parameter is only used in a non-deduced context. [temp.arg.explicit.note]/1 also implies the example is ok. The following change may be an improvement:

If a template parameter that is not a template parameter pack is used only in non-deduced contexts and is not explicitly specified, template argument deduction fails.

Relevant issue: #4555