First issue

[jsk95]

SourceMock/Tests/ArgumentTests.cs

Lines 44 to 51 in c4016d7

	[Fact]
	public void SingleArgument_Optional() {
	var mock = Mock.Of<IMockable>().Get();
	mock.Setup.ParseToInt32().Returns(1);
	Assert.Equal(1, mock.ParseToInt32("x"));
	}

Is the design of the library so that you don't have to specify the shape of the arguments if you don't care?
What happens in the test when the interface has a no param ParseToInt32?

[ashmind]

If there is a clear overload conflict, you will have to specify default explicitly.
I don't have a good constant for Any yet, but default is essentially Any.

So something like this

mock.Setup.ParseToInt32(default).Returns(1); 

[jsk95]

Hm if it's for testing, why not just enforce strictness?

[ashmind]

What do you mean by "strictness"?

[jsk95]

Explicit by default

[ashmind]

Part of the design direction — not keen on passing arguments you don't care about.