Open Radar

 
Summary:
Since @noescape can’t be applied to locals, you can’t write a convenience closure which wraps a @noescape parameter and which trivially never escapes, so you have to duplicate the closure, which increases the surface area for defects.


Steps to Reproduce:
1. Write this code:

enum Type {
	case Unit
	case Function(() -> Type, () -> Type)
	case Sum(() -> Type, () -> Type)

	func reduce<Result>(initial: Result, @noescape _ combine: (Result, Type) -> Result) -> Result {
		let binary: (Type, Type) -> Result = { combine(combine(combine(initial, self), $0), $1) }
		switch self {
		case Unit:
			return combine(initial, self)
		case let Function(t1, t2):
			return binary(t1, t2)
		case let Sum(t1, t2):
			return binary(t1, t2)
		}
	}
}


Expected Results:
I expected it to compile. It’s pretty obvious that `combine` never escapes.


Actual Results:
It failed: `combine` is captured in a closure which may—perhaps by magic—allow it to escape.


Regression:
N/A


Notes:
I also tried @noescape let binary: (Type, Type) -> Result = … but “'noescape' may only be used on 'parameter' declarations,” which is sad-making.