Would you like signs with those chars?

> #define tolower(c) ((_tolower_tab_ + 1)[c])

Even so, I don't believe that the standard *actually* says that c has to be unsigned in that expression - just that the "usual arithmetic conversions" happen (i.e. the compiler magicks it into an int when you're not looking). Compilers presumably added that warning because there were instances of arrays being indexed with negative char, but not negative int or any other signed type. And, again, that presumably had something to do with ASCII supersets and other nonsense involving dirty 7 bit channels.

> Also, EOF is a permitted value and typically -1 (thus the +1 in the above), though that would typically only be an issue for non-C locales.

The argument is of type int (according to the standard, not that untyped macro), not char, so it's completely unambiguous: You are allowed to pass negative numbers to those routines, because int is always signed, and if it is implemented as a macro, it has to accept signed values in the int range. Of course, if you pass negatives other than EOF (or whatever EOF is #define'd to), then the standard presumably gives you UB (which is why it's OK for the array implementation to walk off the end in that case).