Open Radar

 
Summary:
Enabling Whole Module Optimization breaks the linker step of the build. See example project at the following URL for example project that clearly demonstrates this issue:
https://github.com/phatblat/SwiftGenWMO

The type in question (UIColor.Name) is an enum subtype added to UIColor through an extension.

Steps to Reproduce:
1. Clone the above repo
2. Build the “SwiftGenWMO (Release)” scheme

Expected Results:
The build will succeed

Actual Results:
The build fails with the following linker error message:

Ld /Users/ben/Library/Developer/Xcode/DerivedData/SwiftGenWMO-cpguaisqqgbnnhabwlwdrmjpjnmy/Build/Intermediates/SwiftGenWMO.build/Release-iphonesimulator/SwiftGenWMO.build/Objects-normal/i386/SwiftGenWMO normal i386
    cd /Users/ben/dev/ios/SwiftGenWMO
    export IPHONEOS_DEPLOYMENT_TARGET=9.1
    export PATH="/Applications/Xcode7.1.app/Contents/Developer/Platforms/iPhoneSimulator.platform/Developer/usr/bin:/Applications/Xcode7.1.app/Contents/Developer/usr/bin:/usr/local/bin:/usr/bin:/bin:/usr/sbin:/sbin"
    /Applications/Xcode7.1.app/Contents/Developer/Toolchains/XcodeDefault.xctoolchain/usr/bin/clang -arch i386 -isysroot /Applications/Xcode7.1.app/Contents/Developer/Platforms/iPhoneSimulator.platform/Developer/SDKs/iPhoneSimulator9.1.sdk -L/Users/ben/Library/Developer/Xcode/DerivedData/SwiftGenWMO-cpguaisqqgbnnhabwlwdrmjpjnmy/Build/Products/Release-iphonesimulator -F/Users/ben/Library/Developer/Xcode/DerivedData/SwiftGenWMO-cpguaisqqgbnnhabwlwdrmjpjnmy/Build/Products/Release-iphonesimulator -filelist /Users/ben/Library/Developer/Xcode/DerivedData/SwiftGenWMO-cpguaisqqgbnnhabwlwdrmjpjnmy/Build/Intermediates/SwiftGenWMO.build/Release-iphonesimulator/SwiftGenWMO.build/Objects-normal/i386/SwiftGenWMO.LinkFileList -Xlinker -rpath -Xlinker @executable_path/Frameworks -mios-simulator-version-min=9.1 -Xlinker -objc_abi_version -Xlinker 2 -L/Applications/Xcode7.1.app/Contents/Developer/Toolchains/XcodeDefault.xctoolchain/usr/lib/swift/iphonesimulator -Xlinker -add_ast_path -Xlinker /Users/ben/Library/Developer/Xcode/DerivedData/SwiftGenWMO-cpguaisqqgbnnhabwlwdrmjpjnmy/Build/Intermediates/SwiftGenWMO.build/Release-iphonesimulator/SwiftGenWMO.build/Objects-normal/i386/SwiftGenWMO.swiftmodule -Xlinker -dependency_info -Xlinker /Users/ben/Library/Developer/Xcode/DerivedData/SwiftGenWMO-cpguaisqqgbnnhabwlwdrmjpjnmy/Build/Intermediates/SwiftGenWMO.build/Release-iphonesimulator/SwiftGenWMO.build/Objects-normal/i386/SwiftGenWMO_dependency_info.dat -o /Users/ben/Library/Developer/Xcode/DerivedData/SwiftGenWMO-cpguaisqqgbnnhabwlwdrmjpjnmy/Build/Intermediates/SwiftGenWMO.build/Release-iphonesimulator/SwiftGenWMO.build/Objects-normal/i386/SwiftGenWMO

Undefined symbols for architecture i386:
  "direct type metadata for ext.SwiftGenWMO.__ObjC.UIColor.Name", referenced from:
      _globalinit_33_9DB38CA9AF8247CBBD097F938D9B20B4_func0 in ViewController.o
      generic specialization <ext.SwiftGenWMO.__ObjC.UIColor.Name with ext.SwiftGenWMO.__ObjC.UIColor.Name : Swift.Hashable in SwiftGenWMO, Swift.UInt32> of static Swift._VariantDictionaryStorage.maybeGetFromCocoaStorage <A, B where A: Swift.Hashable> (Swift._VariantDictionaryStorage<A, B>.Type)(Swift._CocoaDictionaryStorage, forKey : A) -> B? in ViewController.o
      function signature specialization <Arg[1] = Dead> of generic specialization <(ext.SwiftGenWMO.__ObjC.UIColor.Name, Swift.UInt32)> of static Swift.Array._allocateBufferUninitialized <A> ([A].Type)(Swift.Int) -> Swift._ArrayBuffer<A> in ViewController.o
      function signature specialization <Arg[0] = Owned To Guaranteed, Arg[1] = Dead> of generic specialization <ext.SwiftGenWMO.__ObjC.UIColor.Name with ext.SwiftGenWMO.__ObjC.UIColor.Name : Swift.Hashable in SwiftGenWMO, Swift.UInt32> of static Swift._NativeDictionaryStorage.fromArray <A, B where A: Swift.Hashable> (Swift._NativeDictionaryStorage<A, B>.Type)([(A, B)]) -> Swift._NativeDictionaryStorage<A, B> in ViewController.o
ld: symbol(s) not found for architecture i386
clang: error: linker command failed with exit code 1 (use -v to see invocation)

Ld /Users/ben/Library/Developer/Xcode/DerivedData/SwiftGenWMO-cpguaisqqgbnnhabwlwdrmjpjnmy/Build/Intermediates/SwiftGenWMO.build/Release-iphonesimulator/SwiftGenWMO.build/Objects-normal/x86_64/SwiftGenWMO normal x86_64
    cd /Users/ben/dev/ios/SwiftGenWMO
    export IPHONEOS_DEPLOYMENT_TARGET=9.1
    export PATH="/Applications/Xcode7.1.app/Contents/Developer/Platforms/iPhoneSimulator.platform/Developer/usr/bin:/Applications/Xcode7.1.app/Contents/Developer/usr/bin:/usr/local/bin:/usr/bin:/bin:/usr/sbin:/sbin"
    /Applications/Xcode7.1.app/Contents/Developer/Toolchains/XcodeDefault.xctoolchain/usr/bin/clang -arch x86_64 -isysroot /Applications/Xcode7.1.app/Contents/Developer/Platforms/iPhoneSimulator.platform/Developer/SDKs/iPhoneSimulator9.1.sdk -L/Users/ben/Library/Developer/Xcode/DerivedData/SwiftGenWMO-cpguaisqqgbnnhabwlwdrmjpjnmy/Build/Products/Release-iphonesimulator -F/Users/ben/Library/Developer/Xcode/DerivedData/SwiftGenWMO-cpguaisqqgbnnhabwlwdrmjpjnmy/Build/Products/Release-iphonesimulator -filelist /Users/ben/Library/Developer/Xcode/DerivedData/SwiftGenWMO-cpguaisqqgbnnhabwlwdrmjpjnmy/Build/Intermediates/SwiftGenWMO.build/Release-iphonesimulator/SwiftGenWMO.build/Objects-normal/x86_64/SwiftGenWMO.LinkFileList -Xlinker -rpath -Xlinker @executable_path/Frameworks -mios-simulator-version-min=9.1 -Xlinker -objc_abi_version -Xlinker 2 -L/Applications/Xcode7.1.app/Contents/Developer/Toolchains/XcodeDefault.xctoolchain/usr/lib/swift/iphonesimulator -Xlinker -add_ast_path -Xlinker /Users/ben/Library/Developer/Xcode/DerivedData/SwiftGenWMO-cpguaisqqgbnnhabwlwdrmjpjnmy/Build/Intermediates/SwiftGenWMO.build/Release-iphonesimulator/SwiftGenWMO.build/Objects-normal/x86_64/SwiftGenWMO.swiftmodule -Xlinker -dependency_info -Xlinker /Users/ben/Library/Developer/Xcode/DerivedData/SwiftGenWMO-cpguaisqqgbnnhabwlwdrmjpjnmy/Build/Intermediates/SwiftGenWMO.build/Release-iphonesimulator/SwiftGenWMO.build/Objects-normal/x86_64/SwiftGenWMO_dependency_info.dat -o /Users/ben/Library/Developer/Xcode/DerivedData/SwiftGenWMO-cpguaisqqgbnnhabwlwdrmjpjnmy/Build/Intermediates/SwiftGenWMO.build/Release-iphonesimulator/SwiftGenWMO.build/Objects-normal/x86_64/SwiftGenWMO

Undefined symbols for architecture x86_64:
  "direct type metadata for ext.SwiftGenWMO.__ObjC.UIColor.Name", referenced from:
      _globalinit_33_9DB38CA9AF8247CBBD097F938D9B20B4_func0 in ViewController.o
      generic specialization <ext.SwiftGenWMO.__ObjC.UIColor.Name with ext.SwiftGenWMO.__ObjC.UIColor.Name : Swift.Hashable in SwiftGenWMO, Swift.UInt32> of static Swift._VariantDictionaryStorage.maybeGetFromCocoaStorage <A, B where A: Swift.Hashable> (Swift._VariantDictionaryStorage<A, B>.Type)(Swift._CocoaDictionaryStorage, forKey : A) -> B? in ViewController.o
      function signature specialization <Arg[1] = Dead> of generic specialization <(ext.SwiftGenWMO.__ObjC.UIColor.Name, Swift.UInt32)> of static Swift.Array._allocateBufferUninitialized <A> ([A].Type)(Swift.Int) -> Swift._ArrayBuffer<A> in ViewController.o
      function signature specialization <Arg[0] = Owned To Guaranteed, Arg[1] = Dead> of generic specialization <ext.SwiftGenWMO.__ObjC.UIColor.Name with ext.SwiftGenWMO.__ObjC.UIColor.Name : Swift.Hashable in SwiftGenWMO, Swift.UInt32> of static Swift._NativeDictionaryStorage.fromArray <A, B where A: Swift.Hashable> (Swift._NativeDictionaryStorage<A, B>.Type)([(A, B)]) -> Swift._NativeDictionaryStorage<A, B> in ViewController.o
ld: symbol(s) not found for architecture x86_64
clang: error: linker command failed with exit code 1 (use -v to see invocation)

Regression:
Building the “SwiftGenWMO (Debug)” scheme succeeds with no changes to the code. Likewise, disabling WMO in the Release config and using the “Fast [-O]” value for the SWIFT_OPTIMIZATION_LEVEL build setting also works. It is only when WMO is enabled that these errors occur.