Is T expressions result in CS8121 error if sealed class, implementing a generic interface, is used for T

[DavidArno]

Visual C# 2017 RC 00369-50000-00000-AA146
Visual Studio/15.0.0-RC.3+26127.3:

Steps to Reproduce:

Attempt to compile the following code:

namespace NS1
{
    public interface ITest<T> {}

    internal class Test1 : ITest<int> { }
    internal sealed class Test2 : ITest<int> { }

    internal class C<T>
    {
        public void F(ITest<T> x)
        {
            if (x is Test1 z) { }
            if (x is Test2 y) { }
        }
    }
}

Expected Behavior:
All compiles OK.

Actual Behavior:
The if (x is Test2 y) { } line results in the error:
CS8121 An expression of type ITest<T> cannot be handled by a pattern of type Test2

[controlflow]

Hm, ordinary x is Test2 compiles fine

[DavidArno]

@controlflow,

Indeed, it's only when the new x is T y pattern match is used that the problem occurs and only when the base type is generic and the type in the pattern is sealed.

[DavidArno]

@Pilchie,

I notice you have only tagged this as "Area-Language-Design", rather than as a bug. Does this mean that the x is T y pattern deliberately doesn't support sealed classes? Is there anything documented on why this is the case, if so?

[Pilchie]

Just trying to get it closer to the right people looking at it :)

[aelij]

Also note that the "use pattern matching" analyzer produces an incorrect code fix (IDE0019) that results in non-compilable code in these cases.

[CyrusNajmabadi]

tagging @gafter I believe this is a manifestation of dotnet/csharplang#154, which is something he's aware of and which we're taking a fix for for 7.1. However, i'ts not 100% clear to me this is the same issue. So i'd like him to weigh in.

[gafter]

The simple way to demonstrate that this is the same issue is to see if you get an error when the body of F is rewritten this way:

            //if (x is Test1 z) { }
            var z = (Test1)x;

            //if (x is Test2 y) { }
            var y = (Test2)x;

Indeed, the second cast results in an error.

So this is a dup of dotnet/csharplang#154