I opened up an older project of mine and chose Convert to Objective-C ARC from the Edit/Refactor menu. I am getting the following error:
Apple LLVM compiler 3.0 Error
Error in format of file: /Users/myUserName/Library/Developer/Xcode/DerivedData/ProjectName-fkjvtdsoypoyrdcedtarbtypupor/Build/Intermediates/ProjectName.build/Debug-iphoneos/ProjectName.build/ProjectName-arc.migrate/remap
Is this one of those situations where I need to manually delete some files/folders in Finder and then try it again? I just deleted everything in the "Build" folder of the project, then did Product/Clean and tried it again, but still getting the same error.
Apparently you cannot have the target set to iOS Device when you "Convert to Objective-C ARC"
When I set the target to the simulator, I was able to convert to ARC with no errors.
来源：https://stackoverflow.com/questions/7972960/convert-to-arc-llvm-compiler-3-0-error