Open Radar

 
Summary:
I don’t even know what that means.


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 // this is quite a dull language

	func map<U>(transform: T -> U) -> Expr<U> {
		return .Null
	}
}

func out<Fix: FixpointType>(v: Fix) -> Fix.Algebra {
	return v.out
}

func cata<T, Fix: FixpointType where Fix.Algebra == Expr<Fix>>(f: Expr<T> -> T)(_ term: Fix) -> T {
	return f({ $0.map(cata(f)) }(out(term)))
}


struct Term: FixpointType, CustomDebugStringConvertible {
	init(_ expression: Expr<Term>) {
		out = expression
	}
	let out: Expr<Term>
	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


Actual Results:
but totally does:
Assertion failed: (typeVar != TypeVar && "Cannot be adjacent to oneself"), function getAdjacency, file /Library/Caches/com.apple.xbs/Sources/swiftlang/swiftlang-700.0.38.1/src/swift/lib/Sema/ConstraintGraph.cpp, line 126.
0  swift                    0x000000010e2f8e0b llvm::sys::PrintStackTrace(__sFILE*) + 43
1  swift                    0x000000010e2f954b SignalHandler(int) + 379
2  libsystem_platform.dylib 0x00007fff88493f1a _sigtramp + 26
3  swift                    0x000000010cad63e6 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                    0x000000010c8ce3bd swift::constraints::ConstraintGraphNode::getAdjacency(swift::TypeVariableType*) + 317
7  swift                    0x000000010c8cdd6d swift::constraints::ConstraintGraph::bindTypeVariable(swift::TypeVariableType*, swift::Type) + 301
8  swift                    0x000000010c8da412 swift::constraints::ConstraintSystem::assignFixedType(swift::TypeVariableType*, swift::Type, bool) + 306
9  swift                    0x000000010c927139 swift::constraints::ConstraintSystem::matchTypes(swift::Type, swift::Type, swift::constraints::TypeMatchKind, unsigned int, swift::constraints::ConstraintLocatorBuilder) + 4249
…
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-2afe3d.o 
1.	While type-checking 'cata' at recur.swift:22:1
2.	While type-checking expression at [recur.swift:23:9 - line:23:41] RangeText="f({ $0.map(cata(f)) }(out(term)))"
<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)