Argument constraint That (or Ignored, or _), when nested deeper in A.CallTo, misreports what's being matched

Found while investigating FakeItEasy - How to verify nested arguments value C#.

Consider this test:

public class NestedThat
{
    public class Dto
    {
        public string Id;
    }

    public interface IService
    {
        void DoIt(Dto dto);
    }

    [Test]
    public void Test()
    {
        var fake = A.Fake<IService>();
        fake.DoIt(new Dto { Id = "some id" });

        A.CallTo(() => fake.DoIt(new Dto { Id = A<string>._ })).MustHaveHappened();
    }
}

The author would expect a passing test, but it fails, which isn’t horrible, since the use of A<string>._ inside the Dto constructor is illegal. However, the failure does not lead the reader to the fact that the use of ._ (and I’m sure any variant of That) is bad. Here’s the output:

Test 'FakeItEasyPlay.NestedThat.Test' failed: FakeItEasy.ExpectationException : 

  Assertion failed for the following call:
    FakeItEasyPlay.NestedThat+IService.DoIt(dto: <Ignored>)
  Expected to find it at least once but found it #0 times among the calls:
    1: FakeItEasyPlay.NestedThat+IService.DoIt(dto: FakeItEasyPlay.NestedThat+Dto)

	at FakeItEasy.Core.FakeAsserter.AssertWasCalled(Func`2 callPredicate, String callDescription, Func`2 repeatPredicate, String repeatDescription)
	at FakeItEasy.Configuration.RuleBuilder.MustHaveHappened(Repeated repeatConstraint)
	at FakeItEasy.AssertConfigurationExtensions.MustHaveHappened(IAssertConfiguration configuration)
	NestedThat.cs(19,0): at FakeItEasyPlay.NestedThat.Test()

In addition to not highlighting the illegal ._, we are misreporting the expected argument to DoIt as being equivalent to A<Dto>._.

It may be difficult (impossible) to throw correct error in all cases, but in this one, where we’re capturing a That constraint to match against a parameter of a different type, I’d hope we’d be able to do something.

[thomaslevesque]

It would be nice to update the analyzer to detect this too.

[blairconrad]

This change has been released as part of FakeItEasy 4.1.0.