It.Is is not always implicit when it should be

[kentcb]

This works as expected:

var mock = new SomeMock();
mock.When(x => x.SomeMethod("foo")).Return("bar");

Assert.Equal("bar", mock.SomeMethod("foo"));

This does not:

var mock = new SomeMock();
var obj = new object();
mock.When(x => x.SomeMethod(obj)).Return("bar");

Assert.Equal("bar", mock.SomeMethod(obj));

[kentcb]

Investigation has shown this to be a flaw in the ArgumentFilterVisitor. In some circumstances (such as the second scenario shown above), the object passed into the method invocation is wrapped in a compiler-generated type. Thus, it will be necessary to detect this scenario and "unwrap" the actual object.