[swift-dev] Xcode 8 GM: Compiler segfaults on ExpressibleByArrayLiteral

[Ole Begemann]

I'm seeing a segmentation fault when compiling this code with Xcode 8 GM:

```
// stack.swift
struct Stack<Element> {
     var elements: [Element] = []
}

extension Stack: ExpressibleByArrayLiteral {
     init(arrayLiteral elements: Element...) {
         self.elements = elements
     }
}
```

---

This is my Swift version (Xcode 8 GM):

```
$ xcrun swift --version
Apple Swift version 3.0 (swiftlang-800.0.46.2 clang-800.0.38)
Target: x86_64-apple-macosx10.9
```

I'm on OS X 10.11.6.

---

Compiling with `xcrun swift stack.swift`, I see this:

```
$ xcrun swift stack.swift
0  swift                    0x000000010558fb6d 
PrintStackTraceSignalHandler(void*) + 45
1  swift                    0x000000010558f5b6 SignalHandler(int) + 470
2  libsystem_platform.dylib 0x00007fff8e5a952a _sigtramp + 26
3  libsystem_platform.dylib 0x00007fff5d2ca6d0 _sigtramp + 3469873600
4  swift                    0x0000000102a6d8b9 
swift::irgen::IRGenFunction::emitTypeMetadataRef(swift::CanType) + 73
5  swift                    0x0000000102aab07d void 
llvm::function_ref<void 
(swift::irgen::GenericRequirement)>::callback_fn<(anonymous 
namespace)::EmitPolymorphicArguments::emit(swift::CanTypeWrapper<swift::SILFunctionType>, 
llvm::ArrayRef<swift::Substitution>, swift::irgen::WitnessMetadata*, 
swift::irgen::Explosion&)::$_14>(long, swift::irgen::GenericRequirement) 
+ 861
6  swift                    0x0000000102aa09e1 (anonymous 
namespace)::PolymorphicConvention::enumerateRequirements(llvm::function_ref<void 
(swift::irgen::GenericRequirement)> const&) + 129
7  swift                    0x0000000102aaabdb 
swift::irgen::emitPolymorphicArguments(swift::irgen::IRGenFunction&, 
swift::CanTypeWrapper<swift::SILFunctionType>, 
swift::CanTypeWrapper<swift::SILFunctionType>, 
llvm::ArrayRef<swift::Substitution>, swift::irgen::WitnessMetadata*, 
swift::irgen::Explosion&) + 459
8  swift                    0x0000000102b074f5 (anonymous 
namespace)::IRGenSILFunction::visitFullApplySite(swift::FullApplySite) + 
2997
9  swift                    0x0000000102af2268 
swift::irgen::IRGenModule::emitSILFunction(swift::SILFunction*) + 9080
10 swift                    0x0000000102a184d0 
swift::irgen::IRGenerator::emitLazyDefinitions() + 5216
11 swift                    0x0000000102ad819b 
performIRGeneration(swift::IRGenOptions&, swift::ModuleDecl*, 
swift::SILModule*, llvm::StringRef, llvm::LLVMContext&, 
swift::SourceFile*, unsigned int) + 1723
12 swift                    0x0000000102ad6507 
swift::performIRGeneration(swift::IRGenOptions&, swift::ModuleDecl*, 
swift::SILModule*, llvm::StringRef, llvm::LLVMContext&) + 1527
13 swift                    0x00000001029baa7b 
swift::RunImmediately(swift::CompilerInstance&, 
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&, 
swift::IRGenOptions&, swift::SILOptions const&) + 187
14 swift                    0x00000001029a607e 
performCompile(swift::CompilerInstance&, swift::CompilerInvocation&, 
llvm::ArrayRef<char const*>, int&, swift::FrontendObserver*) + 23358
15 swift                    0x000000010299e265 
swift::performFrontend(llvm::ArrayRef<char const*>, char const*, void*, 
swift::FrontendObserver*) + 17029
16 swift                    0x000000010295b82d main + 8685
17 libdyld.dylib            0x00007fff91eef5ad start + 1
18 libdyld.dylib            0x000000000000000c start + 1846610528
Stack dump:
0.      Program arguments: 
/Applications/Xcode.app/Contents/Developer/Toolchains/XcodeDefault.xctoolchain/usr/bin/swift 
-frontend -interpret stack.swift -target x86_64-apple-macosx10.9 
-enable-objc-interop -sdk 
/Applications/Xcode.app/Contents/Developer/Platforms/MacOSX.platform/Developer/SDKs/MacOSX10.12.sdk 
-color-diagnostics -module-name stack
1.      While emitting IR SIL function 
@_TFV5stack5StackCft12arrayLiteralGSax__GS0_x_ for 'init' at stack.swift:6:5
[1]    38858 segmentation fault  xcrun swift stack.swift
$
```

---

Regression:
I observed the same behavior at least in Xcode 8b6 and b5. I did not 
report this earlier because the problem did not occur in recent Swift 
snapshots. The oldest one I still have installed is 
`DEVELOPMENT-SNAPSHOT-2016-08-07-a`, and with that one it already 
compiles fine:

```
# DEVELOPMENT-SNAPSHOT-2016-08-07-a
$ swift --version
Apple Swift version 3.0-dev (LLVM 17c4c6cf7a, Clang 4ca9e01a7c, Swift 
13a9908f8d)
Target: x86_64-apple-macosx10.9

$ swift ~/code/stack.swift
$
```

---

Workaround:
It compiles fine if I rewrite the `ExpressibleByArrayLiteral` 
initializer like this:

```
struct Stack<Element> {
     var elements: [Element] = []
}

extension Stack: ExpressibleByArrayLiteral {
     init(arrayLiteral elements: Element...) {
         self.init(elements: elements)
     }
}
```

Or alternatively, provide an explicit initializer for the struct:

```
struct Stack<Element> {
     var elements: [Element]
     init(elements: [Element]) {
         self.elements = elements
     }
}

extension Stack: ExpressibleByArrayLiteral {
     init(arrayLiteral elements: Element...) {
         self.elements = elements
     }
}
```

---

Is this something for Radar or bugs.swift.org?

Ole