STINNER Victor <vstin...@python.org> added the comment:
The problem can be simplified to this x.c file: --- static int invalid_day(unsigned int day) { return (day < 0 || day > 6); } int main() { invalid_day(3); return 0; } --- GCC emits the warning: $ gcc x.c -o x -O3 -Wall -Wextra x.c: In function 'invalid_day': x.c:3:17: warning: comparison of unsigned expression in '< 0' is always false [-Wtype-limits] 3 | return (day < 0 || day > 6); | ^ There are different options to avoid the warning: (A) Remove "day < 0" test Easiest option, portable, simple: my PR 23614. (B) Disable compiler warnings on the test Solution currently implemented with pragma + PR 20619 to fix pragmas. (C) Cast the 'day' variable to a signed type I understand that Paul wants the code to be as generic as possible, and not depending on the "day" parameter type. For example, casting to "int8_t" may introduce a risk of integer overflow if day type is larger than 8 bits. Not my favorite option. (D) Make "day < 0" conditional depending if day type is signed or not (E) Check that day type is unsigned to ensure indirectly that "day >= 0" Checking if *a type* is signed or not is easy using the C preprocessor: #define _Py_IS_TYPE_UNSIGNED(type) (((type)-1) > (type)0) The problem is that there is no standard function to get a variable type. GCC and clang provide the __typeof__(var) extension, C++ provides decltype(var) (but CPython code base cannot be built with a C++ compiler if I recall correctly). Paul's PR 20624 introduces Py_ASSERT_VAR_UNSIGNED(var) macro which fails during compilation if the variable is unsigned, or does nothing if the compiler doesn't provide a way to get a variable type (ex: MSC on Windows). -- Most answers about "comparison of unsigned expression always false" question on the Internet are (A): remove the check which emits the warning. My worry is also that outside _zoneinfo.c, they are tons of functions which rely on the fact that an unsigned type cannot be negativ. I don't want to start adding Py_ASSERT_VAR_UNSIGNED(). For me, it's part of the C language and there is no need to be explicit about it. If a developer changes a variable type, they have to check the type bounds and check of the variable is used. I would prefer to be consistent and never check for "< 0" if the type is unsigned, nor ensure with an assertion that the type is unsigned. Paul is in disagreement with that. ---------- _______________________________________ Python tracker <rep...@bugs.python.org> <https://bugs.python.org/issue40686> _______________________________________ _______________________________________________ Python-bugs-list mailing list Unsubscribe: https://mail.python.org/mailman/options/python-bugs-list/archive%40mail-archive.com