Pre-Pitch: Explicit protocol fulfilment with the 'conformance' keyword

Evolution Pitches
14

What about this reunion of our ideas:

The initial pitch was: the conformance keyword looks for statically-known protocol requirements at the location it is used (and errors out if the declaration has no match).

When used inside an extension that declares a protocol conformance, we could have the conformance keyword perform requirement lookup in those protocols only.

To illustrate, consider the following program. It compiles without error with the pitch as written. But it would not with the suggested amendment:

protocol Foo { func foo() }
protocol Bar { func bar() }

struct S { }

extension S: Bar { }
extension S: Foo {
    conformance func foo() { }
    // Initial pitch: no error
    // Amended pitch: function bar() does not fulfill any 'Foo' requirement.
    conformance func bar() { }
}

I'm still not 100% convinced, because I do not like the idea that moving a declaration can make it valid or invalid. Yet this change would help solving some ambiguities that are discussed in the "Known Limits" paragraph of the pitch. The balance is difficult.

Following the same idea, we could have:

extension Foo where Self: Bar {
    // Conformance lookup performed in Foo and Bar only
    conformance func bar() { ... }
}

extension Foo {
    // Conformance lookup performed in Foo and Bar only
    conformance func bar() where Self: Bar { ... }
}

This amendment, if implemented later, would be a breaking change (since some code would stop compiling). This means it must come with the initial proposal, or not at all (making the implementation more difficult). I humbly let the community decide if it's worth it.

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