std::format create undefined-behavior [UndefinedBehaviorSanitizer] [c++20]

[nathan-in-code]

#include <format>

int main()
{
  auto const str = std::format("{0}", -1);
}

If this mini-code is compiled with “AddressSanitizer” option, an undefined-behavior is detected.
The error only occurs with negative Integers.

Clang++-Version = 18
Option = -fsanitize=unsigned-integer-overflow
OS: Ubuntu 24.04 (64-bit)

example:

clang++-18 -fsanitize=unsigned-integer-overflow  -std=c++20 format.cpp -o my_format

output:
runtime error: negation of 4294967295 cannot be represented in type 'make_unsigned_t' (aka 'unsigned int')
SUMMARY: UndefinedBehaviorSanitizer: undefined-behavior /usr/bin/../lib/gcc/x86_64-linux-gnu/14/../../../../include/c++/14/format:1099:12

[philnik777]

Works fine with libc++: https://godbolt.org/z/vn45K3n91

[nathan-in-code]

Ok, then I'll have to install clang-19.

Because clang-18 (default on Ubuntu 24.04) has compiler issues with libc++ at std::formater.

[frederick-vs-ja]

The "bug" can be fixed if we change this line to an equivalent but arcane form.

-	    __u = -static_cast<make_unsigned_t<_Int>>(__i);
+	    __u = make_unsigned_t<_Int>{1} + ~static_cast<make_unsigned_t<_Int>>(__i);

I personally think that -fsanitize=unsigned-integer-overflow is somehow insane. It's now working by design, but I'm not sure whether we should inform libstdc++ to get well with it.