foo.d:

-----
module foo;

import std.datetime : Clock;

class B
{
    void set (Clock clock) { }
}
-----

test.d:

-----
import foo;

class Clock { }
class C : B
{
    override void set (Clock clock) { }
}

void main () { }
-----

$ dmd test.d
> test.d(6,19): Error: function `void test.C.set(Clock clock)` does not override any function, did you mean to override `void foo.B.set(Clock clock)`?

This diagnostic is infuriating. You could easily spend the better part of an hour trying to diagnose what's wrong - the actual issue is that `Clock` refers to two completely different symbols. 

But the compiler is not helpful - it keeps recommending to override the method without giving a hint to what is wrong.