Proposal: allow System.Range to work with extension method Slice(this pseudoreceiver, int, int)

[orthoxerox]

Currently range syntax is incompatible with types like IReadOnlyList<T>, because this type is countable, but doesn't have a Slice(int, int) method.

If I make an extension method IReadOnlyList<T> Slice<T>(this IReadOnlyList<T> source, int start, int length), it's not recognized by the compiler as it looks for an actual instance method on the type.

I propose to extend the lookup algorithm to take extension methods into account.

Alternatives:

• The LDM notes say this can be implemented in a more general way via shapes, but shapes won't arrive until at least C#10.
• An actual default interface method can be added to the BCL on IReadOnlyList<T>, but this will have to be delayed until the next .Net Core release instead of a possible point release of the compiler.