Overload resolution error with indexers across assemblies

[jaredpar]

Repro code

    // lib1.cs 
    public abstract class TaskInput<TKey, TValue>
        where TKey : class
        where TValue : class
    {
        public virtual TValue this[TKey key] => throw null;
    }

    // lib2.cs
    public class CallNode { }
    public sealed class CalleeCallNodeInput<TValue> : TaskInput<CallNode, TValue>
        where TValue : class
    {
        public override TValue this[CallNode key] => throw null;
    }

    // data.cs
    class FunctionSummary { }

    class Repro
    {
        void M()
        {
            CalleeCallNodeInput<FunctionSummary> c = null;
            CallNode node = null;
            var summary = c[node];
        }

        static void Main(){ }
    }

Repro steps:

> csc lib1.cs /t:library
> csc lib2.cs /t:library /r:lib1.dll
> csc data.cs /r:lib1.dll /r:lib2.dll


data.cs(10,27): error CS0121: The call is ambiguous between the following methods or properties: 'TaskInput<TKey, TValue>.this[TKey]' and 'TaskInput<TKey, TValue>.this[TKey]'

This should compile without error or warnings. Likely this is a regession from covariant returns.