[swift-evolution] Allowing `guard let self = self else { … }` for weakly captured self in a closure.

[Hoon H.]

Currently, weakly captured `self` cannot be bound to `guard let …` with same name, and emits a compiler error.

	class Foo {
		func test2(f: ()->()) {
			// … 
		}
		func test1() {
			test2 { [weak self] in
				guard let self = self else { return } // Error.
				print(self)
			}
		}
	}

Do we have any reason to disallow making `self` back to strong reference? It’d be nice if I can do it. Please consider this case.

	class Foo {
		func getValue1() -> Int {
			return 1234
		}
		func test3(value: Int) {
			print(value)
		}
		func test2(f: ()->()) {
			// … 
		}
		func test1() {
			test2 { [weak self] in
				self?.test3(self?.getValue1()) // Doesn't work because it's not unwrapped.

				self!.test3(self!.getValue1()) // Considered harmful due to `!`.

				guard self != nil else { return }
				self!.test3(self!.getValue1()) // OK, but still looks and feels harmful.

				guard let self1 = self else { return }
				self1.test3(self1.getValue1()) // OK, but feels ugly due to unnecessary new name `self1`.

				guard let self = self else { return }
				self.test3(self.getValue1()) // OK.

			}
		}
	}

This also can be applied to `if let` or same sort of constructs.

Even further, we can consider removing required reference to `self` after `guard let …` if appropriate.

	guard let self = self else { return } 
	test3(getValue1()) // Referencing to `self` would not be required anymore. Seems arguable.

I think this is almost fine because users have to express their intention explicitly with `guard` statement. If someone erases the `guard` later, compiler will require explicit self again, and that will prevent mistakes. But still, I am not sure this removal would be perfectly fine.

I am not sure whether this is already supported or planned. But lacked at least in Swift 2.1.1.

— Hoon H.