[SR-9566] Conditional Conformance Crash on type casting

[swift-ci]

Previous ID	SR-9566
Radar	None
Original Reporter	k-o-d-e-n (JIRA User)
Type	Bug

Environment

Xcode 10.1 (10B61)

Additional Detail from JIRA

Votes	0
Component/s	Standard Library
Labels	Bug, RunTimeCrash, Runtime
Assignee	None
Priority	Medium

md5: 7542aad37ab17588adf8a5268320eb15

Issue Description:

I have a similar code in my project.

/// playground
protocol RealtimeValue {}
protocol WritableRealtimeValue: RealtimeValue {}
protocol ChangeableRealtimeValue: RealtimeValue {}
protocol RealtimeCollection: RealtimeValue {}

class _RealtimeValue: RealtimeValue {}


class Object: _RealtimeValue {}


class __RepresentableCollection<Element: RealtimeValue, Ref: RealtimeValue>: _RealtimeValue, RealtimeCollection {}
extension __RepresentableCollection: WritableRealtimeValue where Ref: WritableRealtimeValue {}


struct RCItem: WritableRealtimeValue {}


class References<Element: RealtimeValue>: __RepresentableCollection<Element, RCItem> {}


let _references: _RealtimeValue = __RepresentableCollection<Object, RCItem>()
/// uncomment and crash won't be
//_references as? WritableRealtimeValue


let references: _RealtimeValue = References<Object>()
references as? WritableRealtimeValue // crash

The same happened in regular Xcode project with disabled optimizations.
Crash description:

error: Execution was interrupted, reason: EXC_BAD_ACCESS (code=EXC_I386_GPFLT).
The process has been left at the point where it was interrupted, use "thread return -x" to return to the state before expression evaluation.


* thread #​1, queue = 'com.apple.main-thread', stop reason = EXC_BAD_ACCESS (code=EXC_I386_GPFLT)
  * frame #​0: 0x000000011ac1b3aa libswiftCore.dylib`swift_getObjCClassMetadata + 26
    frame #&#8203;1: 0x000000011ac2ddd9 libswiftCore.dylib`searchInConformanceCache(swift::TargetMetadata<swift::InProcess> const*, swift::TargetProtocolDescriptor<swift::InProcess> const*) + 57
    frame #&#8203;2: 0x000000011ac2d908 libswiftCore.dylib`swift_conformsToProtocolImpl(swift::TargetMetadata<swift::InProcess> const*, swift::TargetProtocolDescriptor<swift::InProcess> const*) + 56
    frame #&#8203;3: 0x000000011ac0c3a6 libswiftCore.dylib`swift::_conformsToProtocol(swift::OpaqueValue const*, swift::TargetMetadata<swift::InProcess> const*, swift::TargetProtocolDescriptor<swift::InProcess> const*, swift::TargetWitnessTable<swift::InProcess> const**) + 38
    frame #&#8203;4: 0x000000011ac2d250 libswiftCore.dylib`swift::_checkGenericRequirements(llvm::ArrayRef<swift::TargetGenericRequirementDescriptor<swift::InProcess> >, std::__1::vector<void const*, std::__1::allocator<void const*> >&, llvm::function_ref<swift::TargetMetadata<swift::InProcess> const* (unsigned int)>, llvm::function_ref<swift::TargetMetadata<swift::InProcess> const* (unsigned int, unsigned int)>) + 416
    frame #&#8203;5: 0x000000011ac2d044 libswiftCore.dylib`swift::TargetProtocolConformanceDescriptor<swift::InProcess>::getWitnessTable(swift::TargetMetadata<swift::InProcess> const*) const + 244
    frame #&#8203;6: 0x000000011ac2e088 libswiftCore.dylib`swift_conformsToProtocolImpl(swift::TargetMetadata<swift::InProcess> const*, swift::TargetProtocolDescriptor<swift::InProcess> const*)::$_3::operator()(swift::TargetProtocolConformanceDescriptor<swift::InProcess> const&, swift::TargetMetadata<swift::InProcess> const*) const + 232
    frame #&#8203;7: 0x000000011ac2db19 libswiftCore.dylib`swift_conformsToProtocolImpl(swift::TargetMetadata<swift::InProcess> const*, swift::TargetProtocolDescriptor<swift::InProcess> const*) + 585
    frame #&#8203;8: 0x000000011ac0c3a6 libswiftCore.dylib`swift::_conformsToProtocol(swift::OpaqueValue const*, swift::TargetMetadata<swift::InProcess> const*, swift::TargetProtocolDescriptor<swift::InProcess> const*, swift::TargetWitnessTable<swift::InProcess> const**) + 38
    frame #&#8203;9: 0x000000011ac120d5 libswiftCore.dylib`_conformsToProtocols(swift::OpaqueValue const*, swift::TargetMetadata<swift::InProcess> const*, swift::TargetExistentialTypeMetadata<swift::InProcess> const*, swift::TargetWitnessTable<swift::InProcess> const**) + 213
    frame #&#8203;10: 0x000000011ac10dc6 libswiftCore.dylib`_dynamicCastToExistential(swift::OpaqueValue*, swift::OpaqueValue*, swift::TargetMetadata<swift::InProcess> const*, swift::TargetExistentialTypeMetadata<swift::InProcess> const*, swift::DynamicCastFlags) + 422
    frame #&#8203;11: 0x000000011dcb621a $__lldb_expr94`main at Untitled Page 2.xcplaygroundpage:26
    frame #&#8203;12: 0x0000000104e705f0 Whats-New-In-Swift-4-2`linkResources + 304
    frame #&#8203;13: 0x00000001066ea62c CoreFoundation`__CFRUNLOOP_IS_CALLING_OUT_TO_A_BLOCK__ + 12
    frame #&#8203;14: 0x00000001066e9de0 CoreFoundation`__CFRunLoopDoBlocks + 336
    frame #&#8203;15: 0x00000001066e4654 CoreFoundation`__CFRunLoopRun + 1284
    frame #&#8203;16: 0x00000001066e3e11 CoreFoundation`CFRunLoopRunSpecific + 625
    frame #&#8203;17: 0x000000010b0dd1dd GraphicsServices`GSEventRunModal + 62
    frame #&#8203;18: 0x000000010bbb381d UIKitCore`UIApplicationMain + 140
    frame #&#8203;19: 0x0000000104e706bd Whats-New-In-Swift-4-2`main + 205
    frame #&#8203;20: 0x000000010810d575 libdyld.dylib`start + 1
    frame #&#8203;21: 0x000000010810d575 libdyld.dylib`start + 1

[belkadan]

@rjmccall, @jckarter, any idea what's going on?

[swift-ci]

Comment by Leonard Stellbrink (JIRA)

Experiences the same with Swift 4.2.

Generic classes that get extended by a protocol with conditional conformance seem to be the reason here.

Casting a object to the protocol will not crash it but when calling a function on the casted object even after making sure that the object is conform to the protocol via a guarded cast will crash at runtime.