Open Radar

 
Summary:
In rdar://problem/22108390, I described allowing `self` to be assigned a value of Optional type in failable value type initializers, automatically returning `nil` if `self` was assigned `nil`.

I personally think that would have better semantics than the semantics of `return` in a failable initializer—it’s inconsistent and confusing that you can return only `nil` but can assign only non-`nil`—but `return` is already established, and assignment to self isn’t a thing for classes anyway, so one might argue that this conflates things unnecessarily.

Therefore I also propose allowing `if let` and `guard let` to assign to `self` in failable value type initializers, which would allow me to replace this:

init?(_ weave: (A -> Location?) -> A -> Location?, _ up: A -> Location?, _ a: A) {
	func into(t1: A) -> Location? {
		return Location(it: t1, down: weave(into), up: up, left: const(nil), right: const(nil))
	}
	guard let location = into(a) else { return nil }
	self = location
}

with the briefer, more idiomatic:

init?(_ weave: (A -> Location?) -> A -> Location?, _ up: A -> Location?, _ a: A) {
	func into(t1: A) -> Location? {
		return Location(it: t1, down: weave(into), up: up, left: const(nil), right: const(nil))
	}
	guard let self = into(a) else { return nil }
}


Steps to Reproduce:
N/A

Expected Results:
N/A

Actual Results:
N/A

Regression:
N/A

Notes:
If you try this today, you get unhelpful error messages:

/Users/rob/Developer/Projects/Manifold/Manifold/Location.swift:64:9: error: pattern matching in a condition requires the 'case' keyword
                guard let self = into(a) else { return nil }
                      ^
                      case 
/Users/rob/Developer/Projects/Manifold/Manifold/Location.swift:64:13: error: binary operator '~=' cannot be applied to operands of type 'Location<A>' and 'Location<A>?'
                guard let self = into(a) else { return nil }
                          ^~~~

Taking its advice and using guard case let self = into(a), you get:

/Users/rob/Developer/Projects/Manifold/Manifold/Location.swift:64:14: warning: 'let' pattern has no effect; sub-pattern didn't bind any variables
                guard case let self = into(a) else { return nil }
                           ^~~~~~~~
                           
/Users/rob/Developer/Projects/Manifold/Manifold/Location.swift:64:18: error: binary operator '~=' cannot be applied to operands of type 'Location<A>' and 'Location<A>?'
                guard case let self = into(a) else { return nil }
                               ^~~~

So you have to bind a pointless temporary either way.