"segmentation fault: 11" when compiling Swift 1.2

HI all,

I'm getting a very frustrating error while compiling Swift 1.2 code with XCode 6.4.

This is the piece of code causing the problem:

let datasource: SPBaseDataSource? = collectionView.dataSource as? SPBaseDataSource

where SPBaseDataSource is a protocol defined as follows:

protocol SPBaseDataSource: UICollectionViewDataSource {
// some methods
}

without that piece of code the projects builds fine, adding that line makes the compiler crash with the following stack:

0  swift                    0x0000000100e772b8 llvm::sys::PrintStackTrace(__sFILE*) + 40
1  swift                    0x0000000100e77794 SignalHandler(int) + 452
2  libsystem_platform.dylib 0x00007fff9231cf1a _sigtramp + 26
3  libsystem_platform.dylib 0x00007fff5f49d1f0 _sigtramp + 3440902896
4  swift                    0x00000001007fc570 (anonymous namespace)::ProtocolPath::findBetterPath(swift::ProtocolDecl*, swift::irgen::ProtocolInfo const&, unsigned int) + 272
5  swift                    0x00000001007fb39a getProtocolWitnessTable(swift::irgen::IRGenFunction&, swift::CanType, swift::irgen::TypeInfo const&, (anonymous namespace)::ProtocolEntry, swift::ProtocolConformance*) + 218
6  swift                    0x00000001007fadab forEachProtocolWitnessTable(swift::irgen::IRGenFunction&, swift::CanType, swift::CanType, llvm::ArrayRef<(anonymous namespace)::ProtocolEntry>, llvm::ArrayRef<swift::ProtocolConformance*>, std::__1::function<void (unsigned int, llvm::Value*)>) + 363
7  swift                    0x00000001007fafd1 swift::irgen::emitOpaqueExistentialContainerInit(swift::irgen::IRGenFunction&, swift::irgen::Address, swift::SILType, swift::CanType, swift::SILType, llvm::ArrayRef<swift::ProtocolConformance*>) + 433
8  swift                    0x0000000100841471 swift::SILVisitor<(anonymous namespace)::IRGenSILFunction, void>::visit(swift::ValueBase*) + 25793
9  swift                    0x000000010083a39d swift::irgen::IRGenModule::emitSILFunction(swift::SILFunction*) + 9901
10 swift                    0x00000001007a5147 swift::irgen::IRGenModule::emitLazyDefinitions() + 151
11 swift                    0x0000000100824cec performIRGeneration(swift::IRGenOptions&, swift::Module*, swift::SILModule*, llvm::StringRef, llvm::LLVMContext&, swift::SourceFile*, unsigned int) + 2268
12 swift                    0x0000000100825693 swift::performIRGeneration(swift::IRGenOptions&, swift::SourceFile&, swift::SILModule*, llvm::StringRef, llvm::LLVMContext&, unsigned int) + 51
13 swift                    0x0000000100761087 frontend_main(llvm::ArrayRef<char const*>, char const*, void*) + 6647
14 swift                    0x000000010075f4e6 main + 1814
15 libdyld.dylib            0x00007fff8d83d5c9 start + 1

Any ideas of how to work around the problem?

thanks

M.