Add nullability info field into discard expression

[TomatorCZ]

Regards the issue and PR.

Problem
Different behavior of discard expression and local variable in out arguments during inferring type argument of generic method.

using System.Diagnostics.CodeAnalysis;

#nullable enable

interface IOperation<T> { }
class StringOperation : IOperation<string?> { }

class C {
    void M() {
        Test(new StringOperation(), out string? discard1); // 1: no warning
        Test(new StringOperation(), out string? _); // 2: CS8620
        Test(new StringOperation(), out var _); // 2: CS8620
        Test(new StringOperation(), out _); // 3: CS8620
    }

    void Test<T>(IOperation<T> operation, [MaybeNull] out T result) => result = default;
}

Reason

BoundDiscardExpression doesn't contain information about nullability resulting in losing information about it between binding and the inference. See code.

Solution

I added field holding examined nullability info.

[jcouv]

Done with review pass (iteration 2)

[TomatorCZ]

I added an indicator if the user defined type of discard expression argument(out _). Then, I consider the argument to be nullable if an user defined it as nullable, or if he omitted the type(e.g. out _, out var _, out string? _). Otherwise I respect user's intention to make the discard non-nullable(e.g. out string _).

As Rikki pointed out, I read the flag in reverse :-/

[TomatorCZ]

Done with fixing the review pass

[jaredpar]

/azp run

[azure-pipelines]

Azure Pipelines successfully started running 2 pipeline(s).

[RikkiGibson]

It wasn't clear to me why we don't just want to call TypeWithAnnotations.Create(type) here. An oblivious annotation seems fine here, just as we have on DeconstructionVariablePendingInference above.

[RikkiGibson]

Consider renaming these to CheckDiscard to match conventions elsewhere in the project.

[RikkiGibson]

Consider assigning default! or similar since these warnings aren't really the interesting part of the test.

[RikkiGibson]

Done review pass. Looking in good shape to me, just a few small things to address.

[TomatorCZ]

Done with fixing review pass

[RikkiGibson]

/azp run

[azure-pipelines]

Azure Pipelines successfully started running 2 pipeline(s).

[RikkiGibson]

@jcouv for a second review

[jcouv]

LGTM Thanks (iteration 10)

[RikkiGibson]

Thanks for the contribution @TomatorCZ!