Fix #4184: Add regression test

[nicolasstucki]

No description provided.

[Blaisorblade]

Not quite; #4184 was about Eq's error, but that particular instance disappeared. We can add that test if there's another reason, but for #4184 a better test might be:

scala> object foo { class Foo }
// defined object foo

scala> object bar { class Foo }
// defined object bar

scala> implicit def eqFoo: Eq[foo.Foo, foo.Foo] = Eq
def eqFoo: Eq[foo.Foo, foo.Foo]

scala> object Bar { new foo.Foo == new bar.Foo }
1 |object Bar { new foo.Foo == new bar.Foo }
  |                 ^^^^^^^^^^^^^^^^^^^^^^
  |      Values of types foo.Foo and bar.Foo cannot be compared with == or !=

[Blaisorblade]

See comment.

Updated the test file

[nicolasstucki]

@Blaisorblade this PR needs a new review