[SR-8019] Conditional conformance typealias segfault

[stephencelis]

Previous ID	SR-8019
Radar	rdar://problem/41216423
Original Reporter	@stephencelis
Type	Bug
Status	Resolved
Resolution	Done

Environment

Swift 4.15 (Xcode 8 beta)

Additional Detail from JIRA

Votes	0
Component/s	Compiler
Labels	Bug, CompilerCrash
Assignee	None
Priority	Medium

md5: 3aa5f0bde3771054f7a549acb48e1558

Issue Description:

The following code segfaults:

struct A<C: Collection> {
  typealias Element = C.Element
}

extension A: Equatable where C: Equatable, Element: Equatable {
  public static func == (lhs: A, rhs: A) -> Bool {
    return true
  }
}

With:

error: associated type 'Element' can only be used with a concrete type or generic parameter base
  typealias Element = Collection.Element
                                 ^
0  swift                    0x000000010af94c5a PrintStackTraceSignalHandler(void*) + 42
1  swift                    0x000000010af94066 SignalHandler(int) + 966
2  libsystem_platform.dylib 0x00007fff7d995d9a _sigtramp + 26
3  swift                    0x00000001087322d6 swift::GenericSignature::enumeratePairedRequirements(llvm::function_ref<bool (swift::Type, llvm::ArrayRef<swift::Requirement>)>) const + 1446
4  swift                    0x00000001086618f0 swift::ASTContext::getConformance(swift::Type, swift::ProtocolDecl*, swift::SourceLoc, swift::DeclContext*, swift::ProtocolConformanceState) + 656
5  swift                    0x00000001086fd51f swift::ConformanceLookupTable::getConformance(swift::NominalTypeDecl*, swift::ConformanceLookupTable::ConformanceEntry*) + 1199
6  swift                    0x000000010879469c swift::NominalTypeDecl::lookupConformance(swift::ModuleDecl*, swift::ProtocolDecl*, llvm::SmallVectorImpl<swift::ProtocolConformance*>&) const + 684
7  swift                    0x0000000108774bd4 swift::ModuleDecl::lookupConformance(swift::Type, swift::ProtocolDecl*) + 1892
8  swift                    0x0000000108745583 swift::GenericSignatureBuilder::lookupConformance(swift::CanType, swift::Type, swift::ProtocolType*) + 227
9  swift                    0x00000001087423b2 swift::GenericSignatureBuilder::addTypeRequirement(llvm::PointerUnion<swift::GenericSignatureBuilder::PotentialArchetype*, swift::Type>, llvm::PointerUnion<swift::GenericSignatureBuilder::PotentialArchetype*, swift::Type>, swift::GenericSignatureBuilder::FloatingRequirementSource, swift::GenericSignatureBuilder::UnresolvedHandlingKind, swift::ModuleDecl*) + 3266
10 swift                    0x000000010874a1cf swift::GenericSignatureBuilder::addRequirement(swift::RequirementRepr const*, swift::GenericSignatureBuilder::FloatingRequirementSource, swift::SubstitutionMap const*, swift::ModuleDecl*) + 1807
11 swift                    0x00000001083b9646 swift::TypeChecker::checkGenericParamList(swift::GenericSignatureBuilder*, swift::GenericParamList*, swift::GenericSignature*, swift::GenericTypeResolver*) + 646
12 swift                    0x00000001083bf45d swift::TypeChecker::checkGenericEnvironment(swift::GenericParamList*, swift::DeclContext*, swift::GenericSignature*, bool, swift::ExtensionDecl*, llvm::function_ref<void (swift::GenericSignatureBuilder&)>, bool) + 1421
13 swift                    0x0000000108382803 swift::TypeChecker::validateExtension(swift::ExtensionDecl*) + 563
14 swift                    0x000000010839dafc (anonymous namespace)::DeclChecker::visit(swift::Decl*) + 332
15 swift                    0x0000000108435616 swift::performTypeChecking(swift::SourceFile&, swift::TopLevelContext&, swift::OptionSet<swift::TypeCheckingFlags, unsigned int>, unsigned int, unsigned int, unsigned int, unsigned int, unsigned int) + 1830
16 swift                    0x000000010806c995 swift::CompilerInstance::performSema() + 4933
17 swift                    0x00000001072678db performCompile(swift::CompilerInstance&, swift::CompilerInvocation&, llvm::ArrayRef<char const*>, int&, swift::FrontendObserver*, swift::UnifiedStatsReporter*) + 731
18 swift                    0x000000010726410f swift::performFrontend(llvm::ArrayRef<char const*>, char const*, void*, swift::FrontendObserver*) + 7839
19 swift                    0x000000010720dc48 main + 13144
20 libdyld.dylib            0x00007fff7d793ee1 start + 1

[belkadan]

Plenty of things wrong with this code, but there should never be a crash!

@swift-ci create

[stephencelis]

It's a reduced case. It should compile just fine, no?

[belkadan]

Well, no. The typealias should say C.Element, not Collection.Element, and the conditional conformance says A: Equatable where A: Equatable, which makes no sense.

[stephencelis]

Sorry those were typos that snuck in when reducing. I've updated the example.

[belkadan]

That version doesn't crash for me. Can you double-check?

[huonw]

As Jordan says, I see a crash with the original:

struct A<C: Collection> { 
    typealias Element = Collection.Element 
} 

extension A: Equatable where A: Equatable, Element: Equatable { 
    public static func == (lhs: NonEmpty, rhs: NonEmpty) -> Bool { 
        return true 
    } 
}

but not with the updated one. If that's the correct reduction, then it's related to https://bugs.swift.org/browse/SR-6569.

[huonw]

I resolved the crash in the original version of the code in #17356 so since we can't reproduce it in the new, I'm going to call this done. Please open another bug if you find the other crash you mention!

[AnnaZaks]

@stephencelis, Could you verify if the problem is fixed and if so move the JIRA to "Closed"?
Thanks!
Anna

[stephencelis]

Yup, looks good! Sorry for the original confusing report 🙂