Swift compile error when subclassing NSObject and using generics

Question

The following Swift code generates a compile error at build time:

import Foundation

class Wrapper<T> : NSObject {
    let obj : T

    init(x : T) {
        self.obj = x
    }
}

Am I doing something wrong or is this a compiler bug?

If so, what could I do to work around it?

The error log:

CompileSwift normal i386 com.apple.xcode.tools.swift.compiler
    cd /Users/hermespique/workspace/HanekeSwift
    export PATH="/Applications/Xcode6-Beta.app/Contents/Developer/Platforms/iPhoneSimulator.platform/Developer/usr/bin:/Applications/Xcode6-Beta.app/Contents/Developer/usr/bin:/usr/bin:/bin:/usr/sbin:/sbin"
    /Applications/Xcode6-Beta.app/Contents/Developer/Toolchains/XcodeDefault.xctoolchain/usr/bin/swift -target i386-apple-ios8.0 -module-name HanekeTests -O0 -sdk /Applications/Xcode6-Beta.app/Contents/Developer/Platforms/iPhoneSimulator.platform/Developer/SDKs/iPhoneSimulator8.0.sdk -g -module-cache-path /Users/hermespique/Library/Developer/Xcode/DerivedData/ModuleCache -I /Users/hermespique/Library/Developer/Xcode/DerivedData/Haneke-dsgqpxenngbaqpctbkhhwtqbiorj/Build/Products/Debug-iphonesimulator -F /Users/hermespique/Library/Developer/Xcode/DerivedData/Haneke-dsgqpxenngbaqpctbkhhwtqbiorj/Build/Products/Debug-iphonesimulator -F /Applications/Xcode6-Beta.app/Contents/Developer/Platforms/iPhoneSimulator.platform/Developer/SDKs/iPhoneSimulator8.0.sdk/Developer/Library/Frameworks -F /Applications/Xcode6-Beta.app/Contents/Developer/Platforms/iPhoneSimulator.platform/Developer/Library/Frameworks -F /Applications/Xcode6-Beta.app/Contents/Developer/Platforms/iPhoneSimulator.platform/Developer/SDKs/iPhoneSimulator8.0.sdk/Developer/Library/Frameworks -c -j4 /Users/hermespique/workspace/HanekeSwift/HanekeTests/XCTestCase+Assert.swift /Users/hermespique/workspace/HanekeSwift/HanekeTests/Cache.swift /Users/hermespique/workspace/HanekeSwift/HanekeTests/Wrapper.swift -output-file-map /Users/hermespique/Library/Developer/Xcode/DerivedData/Haneke-dsgqpxenngbaqpctbkhhwtqbiorj/Build/Intermediates/Haneke.build/Debug-iphonesimulator/HanekeTests.build/Objects-normal/i386/HanekeTests-OutputFileMap.json -serialize-diagnostics -emit-dependencies -emit-module -emit-module-path /Users/hermespique/Library/Developer/Xcode/DerivedData/Haneke-dsgqpxenngbaqpctbkhhwtqbiorj/Build/Intermediates/Haneke.build/Debug-iphonesimulator/HanekeTests.build/Objects-normal/i386/HanekeTests.swiftmodule -Xcc -iquote -Xcc /Users/hermespique/Library/Developer/Xcode/DerivedData/Haneke-dsgqpxenngbaqpctbkhhwtqbiorj/Build/Intermediates/Haneke.build/Debug-iphonesimulator/HanekeTests.build/HanekeTests-generated-files.hmap -Xcc -I/Users/hermespique/Library/Developer/Xcode/DerivedData/Haneke-dsgqpxenngbaqpctbkhhwtqbiorj/Build/Intermediates/Haneke.build/Debug-iphonesimulator/HanekeTests.build/HanekeTests-own-target-headers.hmap -Xcc -ivfsoverlay -Xcc /Users/hermespique/Library/Developer/Xcode/DerivedData/Haneke-dsgqpxenngbaqpctbkhhwtqbiorj/Build/Intermediates/Haneke.build/all-product-headers.yaml -Xcc -iquote -Xcc /Users/hermespique/Library/Developer/Xcode/DerivedData/Haneke-dsgqpxenngbaqpctbkhhwtqbiorj/Build/Intermediates/Haneke.build/Debug-iphonesimulator/HanekeTests.build/HanekeTests-project-headers.hmap -Xcc -I/Users/hermespique/Library/Developer/Xcode/DerivedData/Haneke-dsgqpxenngbaqpctbkhhwtqbiorj/Build/Products/Debug-iphonesimulator/include -Xcc -I/Applications/Xcode6-Beta.app/Contents/Developer/Toolchains/XcodeDefault.xctoolchain/usr/include -Xcc -I/Users/hermespique/Library/Developer/Xcode/DerivedData/Haneke-dsgqpxenngbaqpctbkhhwtqbiorj/Build/Intermediates/Haneke.build/Debug-iphonesimulator/HanekeTests.build/DerivedSources/i386 -Xcc -I/Users/hermespique/Library/Developer/Xcode/DerivedData/Haneke-dsgqpxenngbaqpctbkhhwtqbiorj/Build/Intermediates/Haneke.build/Debug-iphonesimulator/HanekeTests.build/DerivedSources -Xcc -DDEBUG=1 -Xcc -DDEBUG=1 -emit-objc-header -emit-objc-header-path /Users/hermespique/Library/Developer/Xcode/DerivedData/Haneke-dsgqpxenngbaqpctbkhhwtqbiorj/Build/Intermediates/Haneke.build/Debug-iphonesimulator/HanekeTests.build/Objects-normal/i386/HanekeTests-Swift.h

0  swift                    0x000000010a889608 llvm::sys::PrintStackTrace(__sFILE*) + 40
1  swift                    0x000000010a889af4 SignalHandler(int) + 452
2  libsystem_platform.dylib 0x00007fff88e185aa _sigtramp + 26
3  libsystem_platform.dylib 0x00007fff55fb0334 _sigtramp + 3440999844
4  swift                    0x0000000109c87395 swift::irgen::IRGenFunction::emitTypeMetadataRef(swift::CanType) + 21
5  swift                    0x0000000109cbcfb8 (anonymous namespace)::OpaqueArchetypeTypeInfo::destroy(swift::irgen::IRGenFunction&, swift::irgen::Address, swift::CanType) const + 24
6  swift                    0x0000000109cf467b swift::SILVisitor<(anonymous namespace)::IRGenSILFunction, void>::visit(swift::ValueBase*) + 23579
7  swift                    0x0000000109cee266 swift::irgen::IRGenModule::emitSILFunction(swift::SILFunction*) + 8678
8  swift                    0x0000000109c6f6f8 swift::irgen::IRGenModule::emitGlobalTopLevel() + 184
9  swift                    0x0000000109cdb6e3 performIRGeneration(swift::IRGenOptions&, swift::Module*, swift::SILModule*, llvm::StringRef, llvm::LLVMContext&, swift::SourceFile*, unsigned int) + 1859
10 swift                    0x0000000109cdc033 swift::performIRGeneration(swift::IRGenOptions&, swift::SourceFile&, swift::SILModule*, llvm::StringRef, llvm::LLVMContext&, unsigned int) + 51
11 swift                    0x0000000109c4e65a frontend_main(llvm::ArrayRef<char const*>, char const*, void*) + 4842
12 swift                    0x0000000109c4d35d main + 1533
13 libdyld.dylib            0x00007fff8ddbb5fd start + 1
14 libdyld.dylib            0x0000000000000040 start + 1914980932
Stack dump:
0.  Program arguments: /Applications/Xcode6-Beta.app/Contents/Developer/Toolchains/XcodeDefault.xctoolchain/usr/bin/swift -frontend -c /Users/hermespique/workspace/HanekeSwift/HanekeTests/XCTestCase+Assert.swift /Users/hermespique/workspace/HanekeSwift/HanekeTests/Cache.swift -primary-file /Users/hermespique/workspace/HanekeSwift/HanekeTests/Wrapper.swift -enable-objc-attr-requires-objc-module -target i386-apple-ios8.0 -module-name HanekeTests -sdk /Applications/Xcode6-Beta.app/Contents/Developer/Platforms/iPhoneSimulator.platform/Developer/SDKs/iPhoneSimulator8.0.sdk -I /Users/hermespique/Library/Developer/Xcode/DerivedData/Haneke-dsgqpxenngbaqpctbkhhwtqbiorj/Build/Products/Debug-iphonesimulator -F /Users/hermespique/Library/Developer/Xcode/DerivedData/Haneke-dsgqpxenngbaqpctbkhhwtqbiorj/Build/Products/Debug-iphonesimulator -F /Applications/Xcode6-Beta.app/Contents/Developer/Platforms/iPhoneSimulator.platform/Developer/SDKs/iPhoneSimulator8.0.sdk/Developer/Library/Frameworks -F /Applications/Xcode6-Beta.app/Contents/Developer/Platforms/iPhoneSimulator.platform/Developer/Library/Frameworks -F /Applications/Xcode6-Beta.app/Contents/Developer/Platforms/iPhoneSimulator.platform/Developer/SDKs/iPhoneSimulator8.0.sdk/Developer/Library/Frameworks -g -module-cache-path /Users/hermespique/Library/Developer/Xcode/DerivedData/ModuleCache -Xcc -iquote -Xcc /Users/hermespique/Library/Developer/Xcode/DerivedData/Haneke-dsgqpxenngbaqpctbkhhwtqbiorj/Build/Intermediates/Haneke.build/Debug-iphonesimulator/HanekeTests.build/HanekeTests-generated-files.hmap -Xcc -I/Users/hermespique/Library/Developer/Xcode/DerivedData/Haneke-dsgqpxenngbaqpctbkhhwtqbiorj/Build/Intermediates/Haneke.build/Debug-iphonesimulator/HanekeTests.build/HanekeTests-own-target-headers.hmap -Xcc -ivfsoverlay -Xcc /Users/hermespique/Library/Developer/Xcode/DerivedData/Haneke-dsgqpxenngbaqpctbkhhwtqbiorj/Build/Intermediates/Haneke.build/all-product-headers.yaml -Xcc -iquote -Xcc /Users/hermespique/Library/Developer/Xcode/DerivedData/Haneke-dsgqpxenngbaqpctbkhhwtqbiorj/Build/Intermediates/Haneke.build/Debug-iphonesimulator/HanekeTests.build/HanekeTests-project-headers.hmap -Xcc -I/Users/hermespique/Library/Developer/Xcode/DerivedData/Haneke-dsgqpxenngbaqpctbkhhwtqbiorj/Build/Products/Debug-iphonesimulator/include -Xcc -I/Applications/Xcode6-Beta.app/Contents/Developer/Toolchains/XcodeDefault.xctoolchain/usr/include -Xcc -I/Users/hermespique/Library/Developer/Xcode/DerivedData/Haneke-dsgqpxenngbaqpctbkhhwtqbiorj/Build/Intermediates/Haneke.build/Debug-iphonesimulator/HanekeTests.build/DerivedSources/i386 -Xcc -I/Users/hermespique/Library/Developer/Xcode/DerivedData/Haneke-dsgqpxenngbaqpctbkhhwtqbiorj/Build/Intermediates/Haneke.build/Debug-iphonesimulator/HanekeTests.build/DerivedSources -Xcc -DDEBUG=1 -Xcc -DDEBUG=1 -emit-module-doc-path /Users/hermespique/Library/Developer/Xcode/DerivedData/Haneke-dsgqpxenngbaqpctbkhhwtqbiorj/Build/Intermediates/Haneke.build/Debug-iphonesimulator/HanekeTests.build/Objects-normal/i386/Wrapper~partial.swiftdoc -O0 -emit-module-path /Users/hermespique/Library/Developer/Xcode/DerivedData/Haneke-dsgqpxenngbaqpctbkhhwtqbiorj/Build/Intermediates/Haneke.build/Debug-iphonesimulator/HanekeTests.build/Objects-normal/i386/Wrapper~partial.swiftmodule -serialize-diagnostics-path /Users/hermespique/Library/Developer/Xcode/DerivedData/Haneke-dsgqpxenngbaqpctbkhhwtqbiorj/Build/Intermediates/Haneke.build/Debug-iphonesimulator/HanekeTests.build/Objects-normal/i386/Wrapper.dia -emit-dependencies-path /Users/hermespique/Library/Developer/Xcode/DerivedData/Haneke-dsgqpxenngbaqpctbkhhwtqbiorj/Build/Intermediates/Haneke.build/Debug-iphonesimulator/HanekeTests.build/Objects-normal/i386/Wrapper.d -o /Users/hermespique/Library/Developer/Xcode/DerivedData/Haneke-dsgqpxenngbaqpctbkhhwtqbiorj/Build/Intermediates/Haneke.build/Debug-iphonesimulator/HanekeTests.build/Objects-normal/i386/Wrapper.o 
1.  While emitting IR SIL function @_TToFC11HanekeTests7WrapperE for 'deinit' at /Users/hermespique/workspace/HanekeSwift/HanekeTests/Wrapper.swift:11:7
<unknown>:0: error: unable to execute command: Segmentation fault: 11
<unknown>:0: error: swift frontend command failed due to signal (use -v to see invocation)
Command /Applications/Xcode6-Beta.app/Contents/Developer/Toolchains/XcodeDefault.xctoolchain/usr/bin/swift failed with exit code 254

Answer 1

I found a workaround to this problem

class Wrapper<T> {
    let _val: T[]

    init(_ v: T) {
        _val = [v]
    }

    var val : T {
        get {
            return _val[0]
        }
        set {
            _val[0] = newValue
        }
    }
}

Since Array is a reference type (even though it is struct), the size of Wrapper class is now fixed and compiler can handle it.

Answer 2

I think its an X-code bug with generics and NSObject

class Foo<T>:NSObject{
    let _foo:T?

    init(foo:T){
        _foo  = foo
    }
}

gives swiftc failed with exit code 254

Removing the NSObject Works OK. ie:

class Foo<T>{
    let _foo:T?

    init(foo:T){
        _foo  = foo
    }
}

Or adding a constraint to the generic seems to work. ie:

class Foo<T:AnyObject>:NSObject{
    let _foo:T?

    init(foo:T){
        _foo  = foo
    }
}