On OS X top-of-tree Clang+AddressSanitizer, the following produces a false positive
ODR violation during initialization:

    void f() {
        int y = 7;
        dispatch_async(dispatch_get_global_queue(DISPATCH_QUEUE_PRIORITY_BACKGROUND, 0),
^{
            dispatch_sync(dispatch_get_main_queue(), ^{
                printf("num = %d\n", y);
            });
        });
    }

    =================================================================
    ==98053==ERROR: AddressSanitizer: odr-violation (0x00010cb4ae20):
      [1] size=1 'OBJC_CLASS_NAME_2' /tmp/llvm/projects/compiler-rt/test/asan/TestCases/Darwin/objc-odr.mm
      [2] size=1 'OBJC_CLASS_NAME_' /tmp/llvm/projects/compiler-rt/test/asan/TestCases/Darwin/objc-odr.mm
    These globals were registered at these points:
      [1]:
        #0 0x10cb5989c  (/tmp/llvm-cmake-debug/lib/clang/3.6.0/lib/darwin/libclang_rt.asan_osx_dynamic.dylib+0x889c)
        #1 0x10cb4ac2b  (/tmp/llvm-cmake-debug/projects/compiler-rt/test/asan/64bitConfig/TestCases/Darwin/Output/objc-odr.mm.tmp+0x100001c2b)
        #2 0x7fff67e5acea  (<unknown module>)
        #3 0x7fff67e5ae77  (<unknown module>)
      ...

Apparently, something changed in the way Obj-C globals are generated or instrumented.
See the attached test case.