[Proposal]: extension indexers (VS 18.8, .NET 11)

Extension indexers

• Specification: https://github.com/dotnet/csharplang/blob/main/proposals/csharp-15.0/extension-indexers.md
• Discussion: https://github.com/dotnet/csharplang/discussions/9855

Summary

We add the ability to declare extension indexers.

static class E
{
  extension(...)
  {
    int this[int i]
    {
      get { ... }
      set { ... }
    }
  }
}

There are also some other extension improvements we should consider (see cut list from C# 14)

LDM Meetings

• The topic was discussed previously as part of the extension feature in C# 14: https://github.com/dotnet/csharplang/issues/8697
• https://github.com/dotnet/csharplang/blob/main/meetings/2026/LDM-2026-02-02.md#extension-indexers
• https://github.com/dotnet/csharplang/blob/main/meetings/2026/LDM-2026-03-09.md#extension-indexers