Just noticed this legitimate (and verbose) clang output, when compiling nsPresContext.cpp: { ../../../../layout/base/nsPresContext.cpp:421:7: warning: variable 'rv' is used uninitialized whenever 'if' condition is false [-Wsometimes-uninitialized] if (aLanguage && mLangService) { ^~~~~~~~~~~~~~~~~~~~~~~~~ ../../../../layout/base/nsPresContext.cpp:424:17: note: uninitialized use occurs here if (NS_FAILED(rv) || !langGroupAtom) { ~~~~~~~~~~^~~ ../../dist/include/nsError.h:156:67: note: expanded from macro 'NS_FAILED' #define NS_FAILED(_nsresult) ((bool)NS_UNLIKELY(NS_FAILED_impl(_nsresult))) ^ ../../dist/include/nscore.h:385:46: note: expanded from macro 'NS_UNLIKELY' #define NS_UNLIKELY(x) (__builtin_expect(!!(x), 0)) ^ ../../../../layout/base/nsPresContext.cpp:421:3: note: remove the 'if' if its condition is always true if (aLanguage && mLangService) { ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ ../../../../layout/base/nsPresContext.cpp:421:7: warning: variable 'rv' is used uninitialized whenever '&&' condition is false [-Wsometimes-uninitialized] if (aLanguage && mLangService) { ^~~~~~~~~ ../../../../layout/base/nsPresContext.cpp:424:17: note: uninitialized use occurs here if (NS_FAILED(rv) || !langGroupAtom) { ~~~~~~~~~~^~~ ../../dist/include/nsError.h:156:67: note: expanded from macro 'NS_FAILED' #define NS_FAILED(_nsresult) ((bool)NS_UNLIKELY(NS_FAILED_impl(_nsresult))) ^ ../../dist/include/nscore.h:385:46: note: expanded from macro 'NS_UNLIKELY' #define NS_UNLIKELY(x) (__builtin_expect(!!(x), 0)) ^ ../../../../layout/base/nsPresContext.cpp:421:7: note: remove the '&&' if its condition is always true if (aLanguage && mLangService) { ^~~~~~~~~~~~ ../../../../layout/base/nsPresContext.cpp:416:3: note: variable 'rv' is declared here nsresult rv; ^ 2 warnings generated. } from e.g. this opt OS X 10.7 tinderbox build log: https://tbpl.mozilla.org/php/getParsedLog.php?id=16497853&tree=Firefox Relevant code: > 416 nsresult rv; > 417 nsIAtom *langGroupAtom = nullptr; [...] > 421 if (aLanguage && mLangService) { > 422 langGroupAtom = mLangService->GetLanguageGroup(aLanguage, &rv); > 423 } > 424 if (NS_FAILED(rv) || !langGroupAtom) { > 425 langGroupAtom = nsGkAtoms::x_western; // Assume x-western is safe... > 426 } If we don't enter the first clause, then we won't initialize |rv|, and the second clause's "if" condition is bogus. Maybe we want to initialize |rv| to NS_ERROR_FAILURE? Or we could just reverse the condition, since if langGroupAtom is non-null, then rv has presumably been set. (Looks like this chunk of code was added in bug 416581 "part 2" -- marking as dependent on that bug.)

Nice, jdm just landed a fix for this over on bug 813705.