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The motivating example: if (self.weakProp) use(self.weakProp); As with any non-atomic test-then-use, it is possible a weak property to be non-nil at the 'if', but be deallocated by the time it is used. The correct way to write this example is as follows: id tmp = self.weakProp; if (tmp) use(tmp); The warning is controlled by -Warc-repeated-use-of-receiver, and uses the property name and base to determine if the same property on the same object is being accessed multiple times. In cases where the base is more complicated than just a single Decl (e.g. 'foo.bar.weakProp'), it picks a Decl for some degree of uniquing and reports the problem under a subflag, -Warc-maybe-repeated-use-of-receiver. This gives a way to tune the aggressiveness of the warning for a particular project. The warning is not on by default because it is not flow-sensitive and thus may have a higher-than-acceptable rate of false positives, though it is less noisy than -Wreceiver-is-weak. On the other hand, it will not warn about some cases that may be legitimate issues that -Wreceiver-is-weak will catch, and it does not attempt to reason about methods returning weak values. Even though this is not a real "analysis-based" check I've put the bug emission code in AnalysisBasedWarnings for two reasons: (1) to run on every kind of code body (function, method, block, or lambda), and (2) to suggest that it may be enhanced by flow-sensitive analysis in the future. The second (smaller) half of this work is to extend it to weak locals and weak ivars. This should use most of the same infrastructure. Part of <rdar://problem/12280249> llvm-svn: 164854