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The following piece of code crashes the Swift compiler:

enum Event<T> {
	case Value(T)
	case Completion
	case Error(ErrorType)
}

class Stream<T> {
	private let callback: (Event<T> -> Void) -> Void
	
	internal init(callback: (Event<T> -> Void) -> Void) {
		self.callback = callback
	}
	
	func map<U>(transform: T -> U) -> Stream<U> {
		return Stream<U> { sink in
			self.onValue { value in
				sink(transform(value))
			}
		}
	}
	
	func onValue(valueClosure: T -> Void) {
	}
}

The stack trace of the crash is:

Assertion failed: (DC == closure->getParent() && "Decl context isn't correct"), function walkToClosureExprPre, file /Library/Caches/com.apple.xbs/Sources/swiftlang/swiftlang-700.0.52.2/src/swift/lib/Sema/TypeCheckConstraints.cpp, line 673.
0  swift                    0x00000001057cdbbb llvm::sys::PrintStackTrace(__sFILE*) + 43
1  swift                    0x00000001057ce2fb SignalHandler(int) + 379
2  libsystem_platform.dylib 0x00007fff89b3deaa _sigtramp + 26
3  libsystem_platform.dylib 0x00007fff5c46e668 _sigtramp + 3532851160
4  libsystem_c.dylib        0x00007fff906923fb abort + 129
5  libsystem_c.dylib        0x00007fff90659a44 basename + 0
6  swift                    0x0000000103e33021 (anonymous namespace)::PreCheckExpression::walkToExprPre(swift::Expr*) + 609
7  swift                    0x0000000103ef2b9b (anonymous namespace)::Traversal::visit(swift::Expr*) + 107
8  swift                    0x0000000103ef4a00 (anonymous namespace)::Traversal::visitApplyExpr(swift::ApplyExpr*) + 128
9  swift                    0x0000000103ef2c0b (anonymous namespace)::Traversal::visit(swift::Expr*) + 219
10 swift                    0x0000000103ef1ff5 swift::Expr::walk(swift::ASTWalker&) + 53
11 swift                    0x0000000103e298b5 swift::TypeChecker::solveForExpression(swift::Expr*&, swift::DeclContext*, swift::Type, swift::Type, swift::FreeTypeVariableBinding, swift::ExprTypeCheckListener*, swift::constraints::ConstraintSystem&, llvm::SmallVectorImpl<swift::constraints::Solution>&, swift::OptionSet<swift::TypeCheckExprFlags, unsigned int>) + 149
12 swift                    0x0000000103e2f502 swift::TypeChecker::typeCheckExpression(swift::Expr*&, swift::DeclContext*, swift::Type, swift::Type, swift::OptionSet<swift::TypeCheckExprFlags, unsigned int>, swift::ExprTypeCheckListener*) + 450
13 swift                    0x0000000103dc9a00 (anonymous namespace)::FailureDiagnosis::typeCheckChildIndependently(swift::Expr*, bool) + 384
14 swift                    0x0000000103dcb4f0 std::__1::__function::__func<(anonymous namespace)::FailureDiagnosis::visitExpr(swift::Expr*)::$_7, std::__1::allocator<(anonymous namespace)::FailureDiagnosis::visitExpr(swift::Expr*)::$_7>, void (swift::Expr*)>::operator()(swift::Expr*&&) + 32
15 swift                    0x0000000103f567b7 swift::Expr::forEachChildExpr(std::__1::function<void (swift::Expr*)> const&)::ChildWalker::walkToExprPre(swift::Expr*) + 39
16 swift                    0x0000000103ef471f (anonymous namespace)::Traversal::visit(swift::Expr*) + 7151
17 swift                    0x0000000103ef1ff5 swift::Expr::walk(swift::ASTWalker&) + 53
18 swift                    0x0000000103f53b3c swift::Expr::forEachChildExpr(std::__1::function<void (swift::Expr*)> const&) + 44
19 swift                    0x0000000103dc76b9 swift::ASTVisitor<(anonymous namespace)::FailureDiagnosis, bool, void, void, void, void, void>::visit(swift::Expr*) + 9753
20 swift                    0x0000000103dc228b swift::constraints::ConstraintSystem::diagnoseFailureForExpr(swift::Expr*) + 923
21 swift                    0x0000000103dc36e7 swift::constraints::ConstraintSystem::salvage(llvm::SmallVectorImpl<swift::constraints::Solution>&, swift::Expr*, bool) + 4343
22 swift                    0x0000000103e29b30 swift::TypeChecker::solveForExpression(swift::Expr*&, swift::DeclContext*, swift::Type, swift::Type, swift::FreeTypeVariableBinding, swift::ExprTypeCheckListener*, swift::constraints::ConstraintSystem&, llvm::SmallVectorImpl<swift::constraints::Solution>&, swift::OptionSet<swift::TypeCheckExprFlags, unsigned int>) + 784
23 swift                    0x0000000103e2f502 swift::TypeChecker::typeCheckExpression(swift::Expr*&, swift::DeclContext*, swift::Type, swift::Type, swift::OptionSet<swift::TypeCheckExprFlags, unsigned int>, swift::ExprTypeCheckListener*) + 450
24 swift                    0x0000000103dc9a00 (anonymous namespace)::FailureDiagnosis::typeCheckChildIndependently(swift::Expr*, bool) + 384
25 swift                    0x0000000103dc7c24 swift::ASTVisitor<(anonymous namespace)::FailureDiagnosis, bool, void, void, void, void, void>::visit(swift::Expr*) + 11140
26 swift                    0x0000000103dc228b swift::constraints::ConstraintSystem::diagnoseFailureForExpr(swift::Expr*) + 923
27 swift                    0x0000000103dc36e7 swift::constraints::ConstraintSystem::salvage(llvm::SmallVectorImpl<swift::constraints::Solution>&, swift::Expr*, bool) + 4343
28 swift                    0x0000000103e29b30 swift::TypeChecker::solveForExpression(swift::Expr*&, swift::DeclContext*, swift::Type, swift::Type, swift::FreeTypeVariableBinding, swift::ExprTypeCheckListener*, swift::constraints::ConstraintSystem&, llvm::SmallVectorImpl<swift::constraints::Solution>&, swift::OptionSet<swift::TypeCheckExprFlags, unsigned int>) + 784
29 swift                    0x0000000103e2f502 swift::TypeChecker::typeCheckExpression(swift::Expr*&, swift::DeclContext*, swift::Type, swift::Type, swift::OptionSet<swift::TypeCheckExprFlags, unsigned int>, swift::ExprTypeCheckListener*) + 450
30 swift                    0x0000000103dc9a00 (anonymous namespace)::FailureDiagnosis::typeCheckChildIndependently(swift::Expr*, bool) + 384
31 swift                    0x0000000103dc2202 swift::constraints::ConstraintSystem::diagnoseFailureForExpr(swift::Expr*) + 786
32 swift                    0x0000000103dc36e7 swift::constraints::ConstraintSystem::salvage(llvm::SmallVectorImpl<swift::constraints::Solution>&, swift::Expr*, bool) + 4343
33 swift                    0x0000000103e29b30 swift::TypeChecker::solveForExpression(swift::Expr*&, swift::DeclContext*, swift::Type, swift::Type, swift::FreeTypeVariableBinding, swift::ExprTypeCheckListener*, swift::constraints::ConstraintSystem&, llvm::SmallVectorImpl<swift::constraints::Solution>&, swift::OptionSet<swift::TypeCheckExprFlags, unsigned int>) + 784
34 swift                    0x0000000103e2f502 swift::TypeChecker::typeCheckExpression(swift::Expr*&, swift::DeclContext*, swift::Type, swift::Type, swift::OptionSet<swift::TypeCheckExprFlags, unsigned int>, swift::ExprTypeCheckListener*) + 450
35 swift                    0x0000000103e9426a swift::ASTVisitor<(anonymous namespace)::StmtChecker, void, swift::Stmt*, void, void, void, void>::visit(swift::Stmt*) + 4154
36 swift                    0x0000000103e93369 swift::ASTVisitor<(anonymous namespace)::StmtChecker, void, swift::Stmt*, void, void, void, void>::visit(swift::Stmt*) + 313
37 swift                    0x0000000103e92789 (anonymous namespace)::StmtChecker::typeCheckBody(swift::BraceStmt*&) + 25
38 swift                    0x0000000103e9184f swift::TypeChecker::typeCheckFunctionBodyUntil(swift::FuncDecl*, swift::SourceLoc) + 415
39 swift                    0x0000000103e91661 swift::TypeChecker::typeCheckAbstractFunctionBodyUntil(swift::AbstractFunctionDecl*, swift::SourceLoc) + 49
40 swift                    0x0000000103e9238a swift::TypeChecker::typeCheckAbstractFunctionBody(swift::AbstractFunctionDecl*) + 138
41 swift                    0x0000000103e1767c typeCheckFunctionsAndExternalDecls(swift::TypeChecker&) + 220
42 swift                    0x0000000103e184a9 swift::performTypeChecking(swift::SourceFile&, swift::TopLevelContext&, swift::OptionSet<swift::TypeCheckingFlags, unsigned int>, unsigned int) + 1913
43 swift                    0x0000000103c1ecf5 swift::CompilerInstance::performSema() + 4357
44 swift                    0x000000010378e50c performCompile(swift::CompilerInstance&, swift::CompilerInvocation&, llvm::ArrayRef<char const*>, int&) + 748
45 swift                    0x000000010378dfc8 frontend_main(llvm::ArrayRef<char const*>, char const*, void*) + 2728
46 swift                    0x000000010378a02f main + 2015
47 libdyld.dylib            0x00007fff98f965ad start + 1
48 libdyld.dylib            0x0000000000000044 start + 1728486040