[swift-evolution] [Swift 4.0] Conditional conformances via protocol extensions

[Brent Royal-Gordon]

> On Aug 3, 2016, at 10:17 AM, Manav Gabhawala via swift-evolution <swift-evolution at swift.org> wrote:
> 
> I was wondering why this would put any more of a burden on the runtime
> than simple inheritance of protocols. The way this could be
> implemented is to augment the ConformanceTable for nominal types by
> looking up its protocol extension’s inheritance clauses. I can
> definitely see this impacting compile time but I don’t see why runtime
> performance will be any different than simple inheritance. Further,
> cyclic chains can be detected and broken (compiler error) during the
> second pass of semantic analysis.

My understanding—which may be incorrect, by the way—is that the issue is mainly with protocol extensions adding conformances, not specifically with those conformances being conditional, and that it specifically has to do with `is` and `as?` checks across module boundaries.

Suppose you have these declarations in module M:

	public protocol AProtocol {…}
	public protocol BProtocol: AProtocol {…}
	public protocol CProtocol {…}
	
	// Public or otherwise doesn't matter here.
	public struct Foo: BProtocol {…}

Foo essentially has a flat list of the protocols it conforms to attached to it. Notionally, you can think of that list as looking like:

	Foo.self.conformsTo = [BProtocol.self, AProtocol.self]

And when you write `foo is CProtocol`, that eventually translates into:

	foo.dynamicType.conformsTo.contains(CProtocol.self)

For a `Foo`, since the `conformsTo` list doesn't include `CProtocol.self`, it returns `false`.

Now imagine that you write a new module, N, and in it you say:

	extension Foo: CProtocol {…}

You have now retroactively conformed `Foo` to `CProtocol`. Swift needs to reach into module M and add `CProtocol.self` to the `Foo.self.conformsTo` list. This is perfectly doable for a concrete type—it's one flat list, after all.

Instead, though, imagine that module N extended `AProtocol` to add a conformance:

	extension AProtocol: CProtocol {…}

There are two ways to handle this. One is to find all types conforming to `AProtocol`, recursively, and add `CProtocol.self` to their conformance list. The other is to scrap the flat list of conformances and instead make `is` and `as?` recursively search each protocol. Either way, you have replaced a fast, flat operation with a slow, recursive one.

Conditional conformance adds another wrinkle to this, of course—you must not only recursively search the list, but also evaluate the condition to see if it applies in this case. But the general problem of having to replace a fast search with a slow search applies either way.

-- 
Brent Royal-Gordon
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