Draft spec for extended partial methods

[RikkiGibson]

Related to #3301
Proposal: https://github.com/dotnet/csharplang/blob/master/proposals/csharp-9.0/extending-partial-methods.md

C# 9 introduces a new kind of partial method called an extended partial method. An extended partial method must have both a defining declaration and an implementing declaration.

They are required to have explicit accessibility modifiers, which must be identical on both declarations. They are permitted to have out parameters and to have non-void return types. They are permitted to use the virtual, override, sealed, and new modifiers. Both declarations must contain identical combinations of these modifiers, although they may be present in a different order in each declaration.

A single declaration of an extended partial method is said to be an implementing partial declaration if it has a body or if it has an extern modifier. Only the implementing declaration is permitted to have an extern modifier.

[ufcpp]

Can't explicit implementation of an interface method be partial?

interface I
{
    int M();
}

partial class C : I
{
    private partial int I.M(); // requires access modifier but explicit implementation bans it
}

partial class C : I
{
    private partial int I.M() { }
}

(modified public → private)

[HaloFour]

Maybe the explicit implementation syntax needs to optionally allow a private access modifier?

[RikkiGibson]

The feature as shipped in VS 16.7-preview2 does not support partial explicit interface implementations. The compiler team may reconsider this limitation based on user feedback.