nit: this t looks unused #Resolved

To check my understanding, and, in case it helps the second reviewer:

It looks like evaluations.IsEmpty ensures that we don't propose one of our unmatched types as an example pattern, in cases where we would move on to find a more suitable example further along in the process, For example, in test Exhaustiveness_04, this allows us to propose pattern false as an example rather than pattern bool. #Resolved

I didn't follow how it is that we are certain of this. The other two assertions I understand, but, I didn't follow why it is not possible for a pattern matching against an empty union to make it here. #Resolved

I didn't follow how it is that we are certain of this. The other two assertions I understand, but, I didn't follow why it is not possible for a pattern matching against an empty union to make it here.

It is not possible because any type test for an empty Union will result in an ERR_PatternWrongType and, since we are forcing a null check first, we end up with an empty value set after it. However, it is better to not create a value set factory for an empty Union type. I am going to make this change.

Does the prototype comment in _05 cover this diagnostic as well? #Resolved

Does the prototype comment in _05 cover this diagnostic as well?

No. This case is a known behavior of the redundancy checker, that is why it is a hidden diagnostic.
It can be observed in the scenario below, for example. Basically the checker often "complains" if a pattern
always matches, suggesting to use _ instead. The case in _05 is different, replacing the reported
pattern with _ wouldn't be semantically equivalent, hence the comment there.

        [Fact]
        public void Test()
        {
            var src = @"
struct S1
{
    public bool Value => false;
}

class Program
{
    int M(S1 x)
    {
        return x switch {{Value: true} => 1, {Value: false} => 2};
    }
}
";
            var comp = CreateCompilation(src);
            CompileAndVerify(comp).VerifyDiagnostics(
                // (11,54): hidden CS9335: The pattern is redundant.
                //         return x switch {{Value: true} => 1, {Value: false} => 2};
                Diagnostic(ErrorCode.HDN_RedundantPattern, "false").WithLocation(11, 54)
                );
        }

Interesting. Thanks for the explanation, I would not have assumed this was by-design.

Makes sense, I forgot about this behavior.