libstdc++: Fix std::format for pointers [PR110239]

The formatter for pointers was casting to uint64_t which sign extends a
32-bit pointer and produces a value that won't fit in the provided
buffer. Cast to uintptr_t instead.

There was also a bug in the __parse_integer helper when converting a
wide string to a narrow string in order to use std::from_chars on it.
The function would always try to read 32 characters, even if the format
string was shorter than that. Fix that bug, and remove the constexpr
implementation of __parse_integer by just using __from_chars_alnum
instead of from_chars, because that's usable in constexpr even in
C++20.

libstdc++-v3/ChangeLog:

	PR libstdc++/110239
	* include/std/format (__format::__parse_integer): Fix buffer
	overflow for wide chars.
	(formatter<const void*, C>::format): Cast to uintptr_t instead
	of uint64_t.
	* testsuite/std/format/string.cc: Test too-large widths.

2 files changed, 15 insertions, 23 deletions

diff --git a/libstdc++-v3/include/std/format b/libstdc++-v3/include/std/format
index 96a1e62..9d5981e 100644
--- a/libstdc++-v3/include/std/format
+++ b/libstdc++-v3/include/std/format
@@ -269,39 +269,26 @@ namespace __format
       if (__first == __last)
 	__builtin_unreachable();
 
-      // TODO: use this loop unconditionally?
-      // Most integers used for arg-id, width or precision will be small.
-      if (is_constant_evaluated())
-	{
-	  auto __next = __first;
-	  unsigned short __val = 0;
-	  while (__next != __last && '0' <= *__next && *__next <= '9')
-	    {
-	      __val = (__val * 10) + (*__next - '0'); // TODO check overflow?
-	      ++__next;
-	    }
-	  if (__next == __first)
-	    return {0, nullptr};
-	  return {__val, __next};
-	}
-
-      unsigned short __val = 0;
       if constexpr (is_same_v<_CharT, char>)
 	{
-	  auto [ptr, ec] = std::from_chars(__first, __last, __val);
-	  if (ec == errc{})
-	    return {__val, ptr};
-	  return {0, nullptr};
+	  const auto __start = __first;
+	  unsigned short __val = 0;
+	  // N.B. std::from_chars is not constexpr in C++20.
+	  if (__detail::__from_chars_alnum<true>(__first, __last, __val, 10)
+		&& __first != __start) [[likely]]
+	    return {__val, __first};
 	}
       else
 	{
+	  unsigned short __val = 0;
 	  constexpr int __n = 32;
 	  char __buf[__n]{};
-	  for (int __i = 0; __i < __n && __first != __last; ++__i)
+	  for (int __i = 0; __i < __n && (__first + __i) != __last; ++__i)
 	    __buf[__i] = __first[__i];
 	  auto [__v, __ptr] = __format::__parse_integer(__buf, __buf + __n);
 	  return {__v, __first + (__ptr - __buf)};
 	}
+      return {0, nullptr};
     }
 
   template<typename _CharT>
@@ -2118,7 +2105,7 @@ namespace __format
 	typename basic_format_context<_Out, _CharT>::iterator
 	format(const void* __v, basic_format_context<_Out, _CharT>& __fc) const
 	{
-	  auto __u = reinterpret_cast<__UINT64_TYPE__>(__v);
+	  auto __u = reinterpret_cast<__UINTPTR_TYPE__>(__v);
 	  char __buf[2 + sizeof(__v) * 2];
 	  auto [__ptr, __ec] = std::to_chars(__buf + 2, std::end(__buf),
 					     __u, 16);
diff --git a/libstdc++-v3/testsuite/std/format/string.cc b/libstdc++-v3/testsuite/std/format/string.cc
index e421028..d28135e 100644
--- a/libstdc++-v3/testsuite/std/format/string.cc
+++ b/libstdc++-v3/testsuite/std/format/string.cc
@@ -121,6 +121,11 @@ test_format_spec()
   // Invalid presentation types for strings.
   VERIFY( ! is_format_string_for("{:S}", "str") );
   VERIFY( ! is_format_string_for("{:d}", "str") );
+
+  // Maximum integer value supported for widths and precisions is USHRT_MAX.
+  VERIFY( is_format_string_for("{:65535}", 1) );
+  VERIFY( ! is_format_string_for("{:65536}", 1) );
+  VERIFY( ! is_format_string_for("{:9999999}", 1) );
 }
 
 int main()