Parameter pack must be at the end of the parameter list… When and why?

Question

I don\'t get the reason for which a parameter pack must be at the end of the parameter list if the latter is bound to a class, while the constraint is relaxed if the paramet

Answer 1

The first one is not right. The compiler is just buggy and failed to diagnose it. [temp.param]/11:

A template parameter pack of a function template shall not be followed by another template parameter unless that template parameter can be deduced from the parameter-type-list of the function template or has a default argument (14.8.2).

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If the function type T(Args...) is meaningful to the end-user, one way to fix this would be to use a partial specialization instead:

template class C; //undefined
template
class C {
    // implementation
};

Depending on the actual requirements, type-erasing the allocator might also be worth considering.