Getting a CS0570 while debugging a default interface generator method of a variant interface

[mburbea]

Consider the following interface and a dummy class that implements it.

interface I<out T>
{
    System.Collections.Generic.IEnumerable<T> M()
    {
        var x = default(T);
        yield return x;
    }
}

public class C : I<C>
{
    public static void Main() => System.Linq.Enumerable.Any(((I<C>)new C()).M());
}

now put a breakpoint on line 6 (yield return x). Execute the program and you'll notice something odd if you try to watch x, It tells you the following::

error CS0570: '<>x.<>m0(I.d__0)' is not supported by the language

Obviously, the program works so it is supported by the language, but the debugger has no idea what to do with it. This also happens for async (Task{T},ValueTask{T}, or IAsyncEnumerable<T>) returning methods

[Dotnet-GitSync-Bot]

I couldn't figure out the best area label to add to this issue. Please help me learn by adding exactly one area label.

[jeffschwMSFT]

@tommcdon

[tommcdon]

Looks like an expression evaluator issue. @tmat can you help route this?

[tmat]

@tommcdon Labels look good.