[swift-evolution] Require use of override keyword to override dynamically dispatched methods defined in a protocol with a default implementation

[Brent Royal-Gordon]

> Taking inspiration from syntax used for methods in classes that override methods in superclasses, require methods that override dynamically dispatched default implementations in protocol extensions to use the override keyword. Likewise, forbid the override keyword if the method being implemented instead 'masks' (would that be the right word?) a statically dispatched method in a protocol extension which can nonetheless be invoked by upcasting to the protocol.

This has been suggested before, usually in the form of a separate `implement` keyword. The main problem is that it makes it impossible to write a protocol after the fact which formalizes some existing pattern in the types.

What do I mean by that? Well, imagine you need generic access to the `min` and `max` static properties of the various integer types. There's no existing protocol that includes those members. But you can write one and then extend the integer types to conform to your new protocol:

	protocol BoundedIntegerType: IntegerType {
		static var min: Self { get }
		static var max: Self { get }
	}
	extension Int: BoundedType {}
	extension Int8: BoundedType {}
	extension Int16: BoundedType {}
	extension Int32: BoundedType {}
	extension Int64: BoundedType {}

	func printLowestPossibleValueOfValue<Integer: BoundedIntegerType>(x: Integer) {
		print(Integer.min)
	}

This only works because `min` and `max` *don't* need any special marking to be used to satisfy a requirement. Requiring a keyword like you suggest would remove that feature.

-- 
Brent Royal-Gordon
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