...13 PST ---
By git head, the code outputs:

----
test.d(1): Error: interface test.I...

...stdio;
writeln("const");
}

unittest
{
foo(A());
}

DMD outputs the following error:
C:\Users\g.gyolchanyan...

...import std.conv;
static assert(0, "1 != " ~ text(2));

outputs
Error: static assert  ['1',' ','!','=',' ','2']

...Foo[int] map;
  map[1] = Foo();
}

valgrind outputs:
==2562== Conditional jump or move depends on...

...bar = new Bar;
  foo.connect(&bar.handler);
}

outputs:
Error: function signals.Foo.Signal!(int).connect...

...of a real.  In other words it outputs

4
4
16

Instead, what I'd...

...where you specify all valid inputs and outputs for each function (as documentation). There's...

...func;
  dg();

  dg.ptr = cast(void*)b;
  dg();
}

outputs:
A
A

but expected:
A
B

...this:

      auto raw = rawInput();      /* loads data and outputs a struct containing
                                     the array of data...

...getOverloads, test, "foo"))
    {
        alias Tuple!(__traits(getAttributes, o)) attrs;
        pragma(msg, attrs.stringof);
    }
}

Compiler outputs:
()
()