Fix C++17 implementation of wxString::ToCULong()

Do use std::from_chars<unsigned long>() in it as otherwise values
greater than LONG_MAX failed to parse.

Handle negative numbers explicitly to still parse them in this function
as well, as needs to be done for compatibility.

Add a test case for the previously failing numbers.

Closes #23957.

src/common/string.cpp

@@ -1633,20 +1633,34 @@ bool wxString::ToCLong(long *pVal, int base) const
 
 bool wxString::ToCULong(unsigned long *pVal, int base) const
 {
-    // We intentionally don't use std::from_chars() here because this function
-    // is supposed to be compatible with strtoul() and so _succeed_ for "-1",
-    // for example, instead of returning an error as from_chars() (much more
-    // logically) does.
-
     wxCHECK_MSG( pVal, false, "null output pointer" );
 
-    long l;
-    if ( !ToCLong(&l, base) )
+    const wxScopedCharBuffer& buf = utf8_str();
+    auto start = buf.data();
+    const auto end = start + buf.length();
+
+    if ( !SetBaseAndSkipPrefix(base, start, end) )
         return false;
 
-    *pVal = static_cast<unsigned long>(l);
+    // Extra complication: for compatibility reasons, this function does accept
+    // "-1" as valid input (as strtoul() does!), but from_chars() doesn't, for
+    // unsigned values, so check for this separately.
+    if ( *start == '-' )
+    {
+        long l;
+        const auto res = std::from_chars(start, end, l, base);
 
-    return true;
+        if ( res.ec != std::errc{} || res.ptr != end )
+            return false;
+
+        *pVal = static_cast<unsigned long>(l);
+
+        return true;
+    }
+
+    const auto res = std::from_chars(start, end, *pVal, base);
+
+    return res.ec == std::errc{} && res.ptr == end;
 }
 
 bool wxString::ToCDouble(double *pVal) const

tests/strings/strings.cpp

@@ -607,6 +607,17 @@ static const struct ToLongData
     { wxT("0x11"), 17, Number_Ok,       0 },
     { wxT("0x11"),  0, Number_Invalid,  8 },
     { wxT("0x11"), 17, Number_Ok,      16 },
+
+    {
+#if SIZEOF_LONG == 4
+      wxT("0xffffffff"),
+#elif SIZEOF_LONG == 8
+      wxT("0xffffffffffffffff"),
+#else
+    #error "Unknown sizeof(long)"
+#endif
+      (TestValue_t)ULONG_MAX, Number_Unsigned, 0
+    },
 };
 
 wxGCC_WARNING_RESTORE(missing-field-initializers)