MOCK-429: add support for assignable types to Eq matcher

[GrigoriyMikhalkin]

Solution to #429. Adds support for assignable types to Eq matcher.

[cvgw]

Hi @GrigoriyMikhalkin, is this PR ready for review?

[GrigoriyMikhalkin]

@cvgw Hi! Yep.

[cvgw]

I think for aliased types (type A []string) it doesn't matter, but if we are talking about interfaces I think there may be an issue here.

type Foo interface {
  Meow() string
}

type Baz struct {}

func (b Baz) Meow() string {
  return "meow"
}

type Boomer interface {
  Boom(Foo)
}

func TestBoom(t *testing.T) {
  // omitted for brevity
  var bazzer Baz = newBazzer()
  mockBoomer.EXPECT().Boom(gomock.Eq(bazzer)).Times(1)
  var fooer Foo = Foo(bazzer)
  mockBoomer.Boom(fooer) // Foo not assignable to Baz
}

[GrigoriyMikhalkin]

But interface isn't a type(it has underlying type instead). Here is script in playground https://play.golang.org/p/AvDmLq8arUK -- hope i correctly understood your concern.

[cvgw]

Ah ok, I think I understand. In the case of an interface the object will be resolved to its underlying type which must be assignable to the expected type. Thanks for the demo!