[Proposal]: partial extension members

[jcouv]

Specification: https://github.com/dotnet/csharplang/blob/main/proposals/partial-extension-members.md
Discussion: #10228

Summary

Allow partial extension members in extension block in partial enclosing static classes:

public static partial class E
{
    extension(int i)
    {
        public partial void M();
    }
}
public static partial class E
{
    extension(int i)
    {
        public partial void M() { }
    }
}

Design Meetings

• https://github.com/dotnet/csharplang/blob/main/meetings/2026/LDM-2026-07-01.md#partial-extension-members