Open Radar

 
Summary:
The compiler crashes when I destructure & use the elements of a Sliceable constrained to be recursive.


Steps to Reproduce:
1. Write this:

func uncons<S: Sliceable>(sliceable: S) -> (S.Generator.Element, S.SubSlice)? {
	return first(sliceable).map { ($0, dropFirst(sliceable)) }
}

struct Pattern<T, U> {
	let f: T -> U?
}

func cond<T, U, S: Sliceable where S.SubSlice == S, S.Generator.Element == Pattern<T, U>>(x: T, patterns: S) -> U? {
	return uncons(patterns).map { pattern, rest in pattern.f(x) } ?? nil
}


Expected Results:
I expected it to run.


Actual Results:
The compiler crashes.


Regression:
It doesn’t crash if you remove the recursive slice constraint (S.SubSlice == S).

I couldn’t make an iterative version of this due to spurious errors, radar to follow.


Notes:
N/A