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Summary:
I have a function which finds or creates a value of some type from a value of some other type using a lens:

struct Lens<Whole, Part> {
	let from: Whole -> Part
	let to: (Part, Whole) -> Whole
}

func findOrCreate<C: CollectionType, From: Equatable>(collection: C, lens: Lens<From, C.Generator.Element>) -> Lens<From, C.Generator.Element> {
	return Lens(
		from: { value in find(collection) { lens($0, value) == value }.map { collection[$0] } ?? lens(value) },
		to: lens.to)
}

The use case is to find or construct a subview. However, subviews are of type NSView, not of the subclass used by `lens`; so subviews of other types should be ignored. Therefore, I want to change the test to consider only elements of the appropriate subtype of C.Generator.Element. This requires a new type parameter, To, and a subtype constraint To: C.Generator.Element. However, Swift rejects this constraint.


Steps to Reproduce:
1. Compile this:

struct Lens<Whole, Part> {
	let from: Whole -> Part
	let to: (Part, Whole) -> Whole
}

func findOrCreate<C: CollectionType, From: Equatable, To: C.Generator.Element>(collection: C, lens: Lens<From, To>) -> Lens<From, C.Generator.Element> {
	return Lens(
		from: { value in find(collection) { ($0 as? To).map { lens($0, value) == value } ?? false }.map { collection[$0] } ?? lens(value) },
		to: lens.to)
}



Expected Results:
I expected it to compile.


Actual Results:
It does not compile:
Desktop/subtype-constraint.swift:8:55: error: inheritance from non-protocol, non-class type 'C.Generator.Element'


Regression:
N/A


Notes:
I would have accepted that it might want me to constraint C.Generator.Element to AnyObject:

func findOrCreate<C: CollectionType, From: Equatable, To where C.Generator.Element: AnyObject, To: C.Generator.Element>(collection: C, lens: Lens<From, To>) -> Lens<From, C.Generator.Element>

but that doesn’t compile either:

Desktop/subtype-constraint.swift:8:58: error: type 'To' constrained to non-protocol type 'C.Generator.Element'