Adding support for default interface function members

[RexJaeschke]

Re the MS proposal: The main challenge when reading it was the (misleading) occurrence of the modifier override in numerous examples and narrative. As best as I can tell, this modifier is not actually permitted on any interface member. Instead, overriding is achieved via explicit implementation.

There is one open question, which has to do with the topic "Base Interface Invocations," for which the decision "Decided on base(N.I1<T>).M(s)" was made. However, as far as I can tell, support for this was not added to the V8 compiler. Can someone please confirm (or refute) that.

BTW, I think the proposal is misnamed: it really applies to all interface function members (properties, indexers, and events as well), not just methods. (it also allows nested types.)

Note: No spec changes are needed to support entry point Main in an interface. Any non-generic type containing a Main method with void/int return and no/one string[] parameter list is already permitted, which not only allows Main in a struct or class, but now also in an interface.

As of V7, the descriptions of the various member kinds has been located in classes.md, with augmenting and/or overriding text in structs.md, with pointers from structs.md back into classes.md, and that continues. However, now that many members kinds can have implementations in interfaces as well, I've added an intro para to the start of most member sections in classes.md, which contains forward pointers to that member kind's occurrence in structs.md and interfaces.md, as appropriate.

[RexJaeschke]

Depending how PR #680 is resolved, some tweaks might be needed with this PR.

As at 2025-05-03, that PR is still pending, and is tied to the review of this PR.

[BillWagner]

rebased on the latest draft-v8 on 09-23-2023

[KalleOlaviNiemitalo]

Should the references to "C# implementations targeting the CLI" be changed somehow because the ECMA standard CLI does not support default interface members? Possible answers include:

• Remove them; C# 8 implementations cannot target "the CLI".
• Change them to say "targeting a CLI with extensions" or the like.
• Keep them; a C# implementation can still target the standard CLI, although the mapping may become complex or inefficient or have restrictions (like not supporting dynamic loading of assemblies that were not provided at build time).

[RexJaeschke]

Re @KalleOlaviNiemitalo's concern about the Ecma CLI standard not supporting default interface function members, we previously made a related change to the draft-v8 Introduction, which in V7, said:

... Although Microsoft’s implementation of C# relies on CLI for library and run-time support, other implementations of C# need not, provided they support an alternate way of getting at the minimum CLI features required by this C# standard (see Annex C).

It now says:

... Although Microsoft’s implementation of C# relies on CLI for library and run-time support, other implementations of C# need not, provided they support the features and API required by this C# Standard (see Annex C).

I think that allows us to say nothing more for this feature with regard to the CLI.

[gafter]

Suggested change

	Given these definitions, the complete set of types upon which a type depends is the transitive closure of the *directly depends on* relationship.
	Given these definitions, the complete set of types upon which a type depends is the reflexive and transitive closure of the *directly depends on* relationship.

[gafter]

The suggested change is required to disallow class A : A {}

[gafter]

What is "an override"?

[gafter]

I don't recall why this is assigned to me. Is it for general review, or to answer the question about base(T).M()? If the latter, see the following:

• Champion: `base(T)` phase two csharplang#8870
• base(T) - Draft Specification csharplang#2910
• [Proposal]: base(T) phase two csharplang#8871

In short, the base(T).M() syntax has not been added to the language yet.

[BillWagner]

@gafter

I don't recall why this is assigned to me. Is it for general review, or to answer the question about base(T).M()?

Going from memory, but I believe as a general reviewer.