template Bar(A...)
{
    alias A[0] T;
    T[] a = [];
}

template Foo(A...)
{
    alias Bar!(A) bar;
}

alias Foo!(int, 1) foo;
----
Error: can't have array of (int)

T in Bar becomes tuple(int) instead of expected int. The example compiles if all arguments passed to Foo are types:

alias Foo!(byte, int) foo; // ok, foo.a is byte[]