[gcc r12-10228] libstdc++: Fix <charconv> uses of signed types with <bit> functions

[Jonathan Wakely <redi@gcc.gnu.org>]

https://gcc.gnu.org/g:ac0cf0786bb3da60f412afe800fd385686b9517d

commit r12-10228-gac0cf0786bb3da60f412afe800fd385686b9517d
Author: Jonathan Wakely <jwakely@redhat.com>
Date:   Tue Nov 14 10:56:57 2023 +0000

    libstdc++: Fix <charconv> uses of signed types with <bit> functions
    
    In <charconv> we pass the int __base parameter to our internal versions
    of <bit> functions, __bit_width and __countr_zero. Those functions are
    only defined for unsigned types, so we need to convert the base to
    unsigned. The base must be in the range [2,36] so we can mask off the
    low bits and then convert that to unsigned, so that we don't need to
    care about negative values becoming large unsigned values.
    
    libstdc++-v3/ChangeLog:
    
            * include/std/charconv (__from_chars_pow2_base): Convert base to
            unsigned for call to __countr_zero.
            (__from_chars_alnum): Likewise for call to __bit_width.
    
    (cherry picked from commit 1c15303375f7089ff985b085ab877b11ebfbc4b7)

Diff:
---
 libstdc++-v3/include/std/charconv | 6 +++---
 1 file changed, 3 insertions(+), 3 deletions(-)

diff --git a/libstdc++-v3/include/std/charconv b/libstdc++-v3/include/std/charconv
index b4d37435fa0..365a22dab58 100644
--- a/libstdc++-v3/include/std/charconv
+++ b/libstdc++-v3/include/std/charconv
@@ -458,7 +458,7 @@ namespace __detail
 
       // __glibcxx_assert((__base & (__base - 1)) == 0);
       // __glibcxx_assert(_DecOnly ? __base <= 8 : __base <= 32);
-      const int __log2_base = __countr_zero(__base);
+      const int __log2_base = __countr_zero(unsigned(__base & 0x3f));
 
       const ptrdiff_t __len = __last - __first;
       ptrdiff_t __i = 0;
@@ -510,9 +510,9 @@ namespace __detail
     __from_chars_alnum(const char*& __first, const char* __last, _Tp& __val,
 		       int __base)
     {
-      // __glibcxx_assert(!_DecOnly || __base <= 10);
+      // __glibcxx_assert(_DecOnly ? __base <= 10 : __base <= 36);
 
-      const int __bits_per_digit = __bit_width(__base);
+      const int __bits_per_digit = __bit_width(unsigned(__base & 0x3f));
       int __unused_bits_lower_bound = __gnu_cxx::__int_traits<_Tp>::__digits;
       for (; __first != __last; ++__first)
 	{