[Proposal]: Always available extension methods

[333fred]

Always available extension methods

• Specification: [Proposal]: Always available extension methods #8623
• Discussion: [Proposal]: Always available extension methods #8623

Summary

Currently it is impossible to make sure that an extension method is always in scope, except by putting it in the global namespace. We add a step to lookup where we look for extension methods in the same namespace as the receiver type.

Design meetings

• https://github.com/dotnet/csharplang/blob/master/meetings/2020/LDM-2020-11-11.md#always-available-extension-methods