clang -fsanitize=undefined,integer error

[jzmaddock]

When parsing a negative integer, clang triggers an USAN error, test case is simply:

#include <boost/lexical_cast.hpp>

int main()
{
   return boost::lexical_cast<int>("-6575543");
}

Compile with clang++ -fsanitize=undefined,integer and you get:

SUMMARY: UndefinedBehaviorSanitizer: undefined-behavior boost/lexical_cast/detail/converter_lexical_streams.hpp:554:51 in 

This one looks like a real error to me - the code is subtracting two unsigned values, the first of which is zero. A better solution might be to assign the unsigned value to the signed result and then negate, though this would have to account for the INT_MIN special case.

Found while investigating https://github.com/boostorg/multiprecision/issues/313

[pdimov]

Subtracting unsigned values is well defined. -fsanitize=integer diagnoses unsigned integer overflows, which is not undefined behavior per standard. Unsigned integers use modular arithmetic.

[apolukhin]

Double checked the issue. The docs for the sanitizer option say that it is not an undefined behavior, and the sanitizer option is meant to draw attention to a place with potential issue.

Fortunately there is a way to tell sanitizer "yes, I know that therecs no issue here" via __attribute__((no_sanitize("unsigned-integer-overflow"))). I'll add those workarounds soon