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[SR-9967] Runtime crash when calling generic structs with unowned properties #52371

swift-ci opened this issue Feb 21, 2019 · 2 comments


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@swift-ci swift-ci commented Feb 21, 2019

Previous ID SR-9967
Radar None
Original Reporter scott (JIRA User)
Type Bug

Xcode 10.2 beta 3 (10P99q)
Swift compiler swiftlang-1001.0.63.8 clang-1001.0.44

Additional Detail from JIRA
Votes 0
Component/s Compiler
Labels Bug, Runtime
Assignee None
Priority Medium

md5: f92bd658577ff89456d76693b6af0dca

Issue Description:

The following code reliably crashes at runtime:

protocol BarProtocol {}

extension BarProtocol {
    var str: String { return "str" }

struct Foo<T: AnyObject>: BarProtocol {
    unowned let bar: T

class Bar {
    let str2 = "str2"

    var foo: BarProtocol {
        return Foo(bar: self)

_ = Bar().foo.str

with the following stack trace:

0  swift                    0x000000010f514efa PrintStackTraceSignalHandler(void*) + 42
1  swift                    0x000000010f5146dc SignalHandler(int) + 348
2  libsystem_platform.dylib 0x00007fff7c49cb3d _sigtramp + 29
3  libsystem_platform.dylib 0x0000000000000001 _sigtramp + 2209756385
4  libobjc.A.dylib          0x00007fff7b1dcfaa objc_copyWeak + 18
5  libswiftCore.dylib       0x000000011202be3c swift_unknownObjectUnownedCopyInit + 76
6  libswiftCore.dylib       0x0000000110c94365 swift_unknownObjectUnownedCopyInit + 4274423157
7  libswiftCore.dylib       0x0000000110c940a3 swift_unknownObjectUnownedCopyInit + 4274422451
8  swift                    0x000000010bd9d13d llvm::MCJIT::runFunction(llvm::Function*, llvm::ArrayRef<llvm::GenericValue>) + 365
9  swift                    0x000000010bda37fc llvm::ExecutionEngine::runFunctionAsMain(llvm::Function*, std::__1::vector<std::__1::basic_string<char, std::__1::char_traits<char>, std::__1::allocator<char> >, std::__1::allocator<std::__1::basic_string<char, std::__1::char_traits<char>, std::__1::allocator<char> > > > const&, char const* const*) + 1004
10 swift                    0x000000010b3867b1 performCompile(swift::CompilerInstance&, swift::CompilerInvocation&, llvm::ArrayRef<char const*>, int&, swift::FrontendObserver*, swift::UnifiedStatsReporter*) + 54977
11 swift                    0x000000010b375ac7 swift::performFrontend(llvm::ArrayRef<char const*>, char const*, void*, swift::FrontendObserver*) + 6823
12 swift                    0x000000010b31590d main + 1261
13 libdyld.dylib            0x00007fff7c2b1ed9 start + 1

Ways to make the crash go away:

  • Remove `Bar.str2` (the entire property)

  • Make the struct non-generic

  • Change the struct to a class

  • Removed `unowned` (to weak or strong)

We have seen a handful of reports of this happening with current tooling (Xcode 10.1) as well, but can't reliably reproduce.

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@belkadan belkadan commented Feb 21, 2019

@mikeash, does this sound familiar?

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@mikeash mikeash commented Feb 21, 2019

That's a new one for me. Pretty interesting crash. The reproducer crashes on a recent build from master. Looks like disagreement as to whether this reference should use Swift or ObjC unowned semantics.

@swift-ci swift-ci transferred this issue from apple/swift-issues Apr 25, 2022
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