[loeg]

GCC has had -Wsign-compare:

       -Wsign-compare
           Warn when a comparison between signed and unsigned values could
           produce an incorrect result when the signed value is converted to
           unsigned.  In C++, this warning is also enabled by -Wall.  In C, it
           is also enabled by -Wextra.

Since 1996. https://github.com/gcc-mirror/gcc/commit/de9554eb8ae74764555...

(And prior to that explicit option, it was just enabled as part of -Wextra/-W, from 1995: https://github.com/gcc-mirror/gcc/commit/7030c69607d547b3227... . For comparison, Google was founded in 1996.)

I buy that some of the integer promotion rules really suck, and Google's founders learned C prior to 1996. But the compiler has been able to warn us about this particular footgun for a long time.