[PATCH] libstdc++: Make __from_chars_alnum_to_val conversion explicit

[Marco Falke]

(in reply to https://gcc.gnu.org/pipermail/gcc-patches/2022-July/598412.html,
adding libstdc++ to CC, with the same patch attached again)

To clarify, this is not a fix for a user-facing issue of gcc or a fix
for UB. It is just a minor UX improvement for developers that use the
clang integer sanitizer to detect implicit int conversions.

To reproduce:

$ cat 1.cpp
#include <charconv>

int main() {
  const auto a{"-1"};
  unsigned b{};
  std::from_chars(a, a + 2, b);
}
$ clang++ -fsanitize=integer -std=c++17 1.cpp -o exe && ./exe
/usr/bin/../lib64/gcc/x86_64-s
use-linux/12/../../../../include/c++/12/charconv:439:9:
runtime error: implicit conversion from type 'int' of value -3
(32-bit, signed) to type 'unsigned char' changed the value to 253
(8-bit, unsigned)
SUMMARY: UndefinedBehaviorSanitizer: undefined-behavior
/usr/bin/../lib64/gcc/x86_64-suse-linux/12/../../../../include/c++/12/charconv:439:9

Best,
Marco
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