Test case:
----
void foo(const int x)
{
    int func(ref int) { return 1; }
    func(x);
    // Error: function test.foo.func (ref int x) is not callable
    // using argument types (const(int))
    / --> OK

    int delegate(ref int) dg = (ref int x) => 1;
    dg(x);
    // --> compiles without error, bad!

    int function(ref int) fp = (ref int x) => 1;
    fp(x);
    // --> compiles without error, bad!
}