foreach on non-generic should not issue nullable warnings

[stephentoub]

Consider:

#nullable enable
using System.Collections;
using System.Collections.Generic;

public class C {
    public void M()
    {
        var c = new BarCollection();
        foreach (Bar b in c) { } // warning CS8606: Possible null reference assignment to iteration variable
        foreach (Zoo? z in c) { } // ok
    }
}

public class BarCollection : IEnumerable, IEnumerable<Bar>
{
    public IEnumerator GetEnumerator() => null!;
    IEnumerator<Bar> IEnumerable<Bar>.GetEnumerator() => null!;
}

public class Bar{}
public class Zoo{}

The foreach (Bar b in c) will incur a nullable warning, because IEnumerator.Current is typed to return object?. However, foreach over a non-generic IEnumerable allows the iteration variable to be of any type, without warning, as highlighted by the foreach (Zoo? z in c) which has no warnings, even though it'll blow up at run time due to a failed cast.

We should change the nullable heuristics to simply not warn for the iteration variable when foreach binds to the non-generic IEnumerable.

Updated proposal: could we special-case for nullability the case where the type being enumerated implements IEnumerable<T>, and if the dev specifies T as the iteration variable or anything in its inheritance hierarchy, and if T is defined as non-nullable, treating it as non-null?

cc: @agocke, @jcouv

[gafter]

We could consider modifying the library so the type of IEnumerator.Current is oblivious.

[jnm2]

This would solve dotnet/csharplang#3045.

[stephentoub]

Decision here is to change IEnumerator.Current to be oblivious in the interface definition.

[danmoseley]

#37969

[stephentoub]

Fixed by #37969