Open Radar

 
Summary:
This is another one where the compiler correctly reports that it can’t compile but then crashes trying anyway


Steps to Reproduce:
1. This:
protocol FixpointType {
	typealias Algebra : AlgebraicType
	init(_ : Algebra)
	var out: Algebra { get }}
protocol AlgebraicType {typealias Recur}
enum Expr<T>: AlgebraicType {
	typealias Recur = T
	case Null
	func map<U>(transform: T -> U) -> Expr<U> {return .Null}
}
func out<Fix: FixpointType>(v: Fix) -> Fix.Recur {return v.out}
func cata<T, Fix: FixpointType where Fix.Recur == Expr<Fix>>(f: Expr<T> -> T)(_ term: Fix) -> T {
	return f({ $0.map(cata(f)) }(out(term)))
}
struct Term: FixpointType, CustomDebugStringConvertible {
	typealias Algebra = Expr<Term>
	var debugDescription: String {return cata(Term.toDebugString)(self)}
	static func toDebugString(expression: Expr<String>) -> String {return ""}
}


Expected Results:
should not crash the compiler

Actual Results:
but totally does:
…errors…
…
3  swift                    0x0000000100b503e6 swift::Type::transform(std::__1::function<swift::Type (swift::Type)> const&) const + 54
4  libsystem_c.dylib        0x00007fff86f90b53 abort + 129
5  libsystem_c.dylib        0x00007fff86f58c39 basename + 0
6  swift                    0x0000000100b45c9f swift::Substitution::Substitution(swift::ArchetypeType*, swift::Type, llvm::ArrayRef<swift::ProtocolConformance*>) + 271
…
Stack dump:
0.	Program arguments: /Applications/Xcode-7-beta.app/Contents/Developer/Toolchains/XcodeDefault.xctoolchain/usr/bin/swift -frontend -c -primary-file recur.swift -target x86_64-apple-darwin14.3.0 -enable-objc-interop -module-name recur -o /var/folders/9y/f7n4rvx901nctm_5d16c1wm40000gn/T/recur-42ee18.o 
1.	While type-checking getter for debugDescription at recur.swift:17:31
2.	While type-checking expression at [recur.swift:17:39 - line:17:68] RangeText="cata(Term.toDebugString)(self)"
<unknown>:0: error: unable to execute command: Abort trap: 6
<unknown>:0: error: swift frontend command failed due to signal (use -v to see invocation)