[committed] libstdc++: Fix uses of signed types with functions

[Jonathan Wakely]

Tested x86_64-linux. Pushed to trunk. Backports needed too.

-- >8 --

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.
---
 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 6cb7fecff87..54632968f5a 100644
--- a/libstdc++-v3/include/std/charconv
+++ b/libstdc++-v3/include/std/charconv
@@ -465,7 +465,7 @@ 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;
@@ -517,9 +517,9 @@ 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)
        {
-- 
2.41.0