This example compiles:

    template <typename... Args, typename T>
    void foo(Args..., T ) { }

    int main() {
        foo(0);
    }

with Args deducing as empty and T as int. But Args is a non-deduced context (http://eel.is/c++draft/temp#deduct.type-5.7). And I don't think "trailing" can be said to apply to Args here (http://eel.is/c++draft/temp#arg.explicit-3.sentence-2). The call should fail.