[committed] libstdc++: Use unsigned char argument to std::isdigit

[Jonathan Wakely]

Passing plain char to isdigit is undefined if the value is negative.

libstdc++-v3/ChangeLog:

	* include/std/charconv (__from_chars_alnum): Pass unsigned
	char to std::isdigit.

Tested powerpc64le-linux. Committed to trunk.

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commit d0d6ca019717305df0ef41e3fe1da48f7f561fac
Author: Jonathan Wakely <jwakely@redhat.com>
Date:   Wed May 5 11:19:55 2021

    libstdc++: Use unsigned char argument to std::isdigit
    
    Passing plain char to isdigit is undefined if the value is negative.
    
    libstdc++-v3/ChangeLog:
    
            * include/std/charconv (__from_chars_alnum): Pass unsigned
            char to std::isdigit.

diff --git a/libstdc++-v3/include/std/charconv b/libstdc++-v3/include/std/charconv
index 193702e677a..571be075a6b 100644
--- a/libstdc++-v3/include/std/charconv
+++ b/libstdc++-v3/include/std/charconv
@@ -565,7 +565,7 @@ namespace __detail
       while (__first != __last)
 	{
 	  unsigned char __c = *__first;
-	  if (std::isdigit(__c))
+	  if (std::isdigit(static_cast<unsigned char>(__c)))
 	    __c -= '0';
 	  else
 	    {