Open Radar

 
Summary:
Swift can’t type Functor accurately as a protocol, so I’m trying to define a Functor struct that can be returned by other types. My first stab at this results in broken codegen.


Steps to Reproduce:
1. Compile this:
infix operator >>> { associativity right precedence 170 }
public protocol Relatable {
	typealias To
}

public struct Functor<F: Relatable, G: Relatable> {
	public init(_ out: F, _ map: F -> (F.To -> G.To) -> G) {
		self.out = out
		self.map = map
	}

	public func map(transform: F.To -> G.To) -> G {
		return map(out)(transform)
	}

	private let out: F
	private let map: F -> (F.To -> G.To) -> G
}

public protocol FixpointType {
	typealias F
	init(F)
	static func out(Self) -> F
}

func flip<A, B, C>(f: A -> B -> C) -> B -> A -> C {
	return { b in { a in f(a)(b) } }
}

func >>> <A, B, C> (left: A -> B, right: B -> C) -> A -> C {
	return { right(left($0)) }
}

public func cata<T, Fix: FixpointType, F: Relatable, G: Relatable where Fix.F == Functor<F, G>, F.To == Fix, G.To == T>(f: G -> T)(_ term: Fix) -> T {
	return (Fix.out >>> (flip(Functor.map)(cata(f))) >>> f)(term)
}


Expected Results:
I expected it to compile.


Actual Results:
The compiler bails early:
Call parameter type does not match function signature!
  %2 = bitcast %V7functor7Functor.16* %0 to %V7functor7Functor.18*
 %V7functor7Functor.17*  %20 = tail call { i8*, %swift.refcounted* } @_TTRG3_RPq0_P7functor12FixpointType_Pq1_PS_9Relatable_Pq2_PS1__Eqq0_1FGVS_7Functorq1_q2__Eqq1_2Toq0_Eqq2_2Toq__XFo_iGS2_q1_q2___oXFo_oXFo_iq0__iq___iq2___XFo_iGS2_q1_q2___oXFo_iXFo_iq0__iq___iq2___(%V7functor7Functor.18* noalias %2, i8* %.fn.load, %swift.refcounted* %19, %swift.type* %T, %swift.type* %Fix, i8** %9, %swift.type* %F, i8** %13, %swift.type* %G, i8** %17)
LLVM ERROR: Broken function found, compilation aborted!

Note that this error is nowhere to be found in Xcode, it’s only visible from the CLI.


Regression:
One workaround is to rewrite the function without using function composition. Bleah.


Notes:
N/A