Don't be a fool and wonder why nothing is working... ADD THIS CHECK.

ARCCheck.m

// ARC is compatible with iOS 4.0 upwards, but you need at least Xcode 4.2 with Clang LLVM 3.0 to compile it.
#if !defined(__clang__) || __clang_major__ < 3 || !__has_feature(objc_arc)
#error This project must be compiled with ARC (Xcode 4.2+ with LLVM 3.0 and above)
#endif

The recommended idiom for checking features is:

#ifndef __has_feature         // Optional of course.
  #define __has_feature(x) 0  // Compatibility with non-clang compilers.
#endif

i.e., there’s no need to check for Clang specifically. Having done that, one can safely use __has_feature(), e.g.

#if !__has_feature(objc_arc)
  #error …
#endif