Detect wrong argument constraint type at runtime

[thomaslevesque]

Related to #1177, #1230, #1163

We already have the FakeItEasy0005 diagnostic to detect this, but we can also detect it at runtime, and we should throw in this case.

A possible approach would be to examine all the constraints collected by IArgumentConstraintTrapper.TrapConstraints and check if their type is compatible with the parameter type.

[blairconrad]

I like the idea of simply rejecting all expressions that are conversions in CheckArgumentExpressionIsValid.
Sadly, we already allow the user to pass a T constraint in the place of a Nullable<T> parameter.

@thomaslevesque may be right: we might need to investigate the types that we get from the constraint trap.
Of course, we could just do this when the expression is a conversion. I'll play a bit.

[thomaslevesque]

From #1230:

True. I have started working on #1237

Oh. So have I. It was unowned, so I self-assigned and started working on it last night.

As you like @blairconrad. I'm mostly done, but I haven't written any tests yet, and I won't have time to work on it for a few days. If you want to look at what I have, here it is: thomaslevesque@41e6011

I added a new Type property to IArgumentConstraint in order to be able to compare against the actual parameter type.

[blairconrad]

Oh, man. My approach was going to be totally different. Which probably means I'm forgetting lots of stuff! I don't mind just noodling along as a hobby even if/when we take your fix.

I'm going to be away tomorrow and then Friday through Sunday as well, so who knows when I'll be done…

[thomaslevesque]

My approach was going to be totally different

I look forward to seeing what you had in mind ;)

[blairconrad]

Well, here it is. Very rough. Tests at least need review and the production code needs refactor.
blairconrad@99b6a80

[thomaslevesque]

It's an interesting approach. I like the fact that it doesn't require as much changes as mine. On the other hand, it doesn't detect wrong types in hidden nested constraints, e.g. constraints created by user-defined methods.

[blairconrad]

Can you gimme an example of such a thing?

[thomaslevesque]

Something like this:

public static class Extensions
{
    public static int IsEven(this IArgumentConstraintManager<int> manager)
    {
        return manager.Matches(x => x % 2 == 0);
    }
}

Or even:

private static int AnEvenNumber()
{
    return A<int>.That.Matches(x => x % 2 == 0);
}

[blairconrad]

Ugh. You're right, of course. I need to look at your solution more closely to see how it works. I have to relearn argument constraint trapping every single time I look at it.

[thomaslevesque]

I need to look at your solution more closely to see how it works. I have to relearn argument constraint trapping every single time I look at it.

You don't really need to understand it... I didn't touch how constraints are trapped, I'm just examining the results.

[blairconrad]

I found my refresher to be useful anyhow. Your approach has more promise. Abandoning mine.

[blairconrad]

This change has been released as part of FakeItEasy 4.2.0.