[libc++] Fix wraparound issue with -fsanitize=integer in string operator>> (#106263)

Fixes #106261
rdar://133991190

Author: ldionne

libcxx/include/__type_traits/make_unsigned.h

@@ -86,12 +86,10 @@ template <class _Tp>
 using make_unsigned_t = __make_unsigned_t<_Tp>;
 #endif
 
-#ifndef _LIBCPP_CXX03_LANG
 template <class _Tp>
-_LIBCPP_HIDE_FROM_ABI constexpr __make_unsigned_t<_Tp> __to_unsigned_like(_Tp __x) noexcept {
+_LIBCPP_HIDE_FROM_ABI _LIBCPP_CONSTEXPR __make_unsigned_t<_Tp> __to_unsigned_like(_Tp __x) _NOEXCEPT {
   return static_cast<__make_unsigned_t<_Tp> >(__x);
 }
-#endif
 
 template <class _Tp, class _Up>
 using __copy_unsigned_t = __conditional_t<is_unsigned<_Tp>::value, __make_unsigned_t<_Up>, _Up>;

libcxx/include/istream

@@ -165,6 +165,7 @@ template <class Stream, class T>
 #include <__type_traits/conjunction.h>
 #include <__type_traits/enable_if.h>
 #include <__type_traits/is_base_of.h>
+#include <__type_traits/make_unsigned.h>
 #include <__utility/declval.h>
 #include <__utility/forward.h>
 #include <bitset>
@@ -1211,12 +1212,17 @@ operator>>(basic_istream<_CharT, _Traits>& __is, basic_string<_CharT, _Traits, _
     try {
 #endif
       __str.clear();
-      streamsize __n = __is.width();
-      if (__n <= 0)
-        __n = __str.max_size();
-      if (__n <= 0)
-        __n = numeric_limits<streamsize>::max();
-      streamsize __c            = 0;
+      using _Size              = typename basic_string<_CharT, _Traits, _Allocator>::size_type;
+      streamsize const __width = __is.width();
+      _Size const __max_size   = __str.max_size();
+      _Size __n;
+      if (__width <= 0) {
+        __n = __max_size;
+      } else {
+        __n = std::__to_unsigned_like(__width) < __max_size ? static_cast<_Size>(__width) : __max_size;
+      }
+
+      _Size __c                 = 0;
       const ctype<_CharT>& __ct = std::use_facet<ctype<_CharT> >(__is.getloc());
       while (__c < __n) {
         typename _Traits::int_type __i = __is.rdbuf()->sgetc();