QHash: make MSVC happy about the iterators passed to is_permutation

MSVC warns about iterators being passed to certain Standard Library
algorithms. dbd55cdaf3 introduced
a usa of std::is_permutation in a public header, which is causing
such a warning to be emitted.

To suppress the warning, Microsoft suggests to either use the 4-arg
std::is_permutation overload (which however is not available
in MSVC 2013) or to use a Standard Library extension, which we are
already using elsewhere in Qt to deal with the same problem. However,
that extension requires the iterator to be moved by size_t quantities,
which isn't the case for QHash::iterator, and therefore generates
more warnings about loss of precision (size_t -> int).

Therefore, go with the 4-arg std::is_permutation, only on MSVC >= 2015.

Change-Id: Idfcff28d14e0f1fde5d77f1deb9eec27c87ff5cd
Task-number: QTBUG-61902
Reviewed-by: Thiago Macieira <thiago.macieira@intel.com>

src/corelib/tools/qhash.h

@@ -950,8 +950,22 @@ Q_OUTOFLINE_TEMPLATE bool QHash<Key, T>::operator==(const QHash &other) const
             return false;
 
         // Keys in the ranges are equal by construction; this checks only the values.
-        if (!std::is_permutation(it, thisEqualRangeEnd, otherEqualRange.first))
+        //
+        // When using the 3-arg std::is_permutation, MSVC will emit warning C4996,
+        // passing an unchecked iterator to a Standard Library algorithm. We don't
+        // want to suppress the warning, and we can't use stdext::make_checked_array_iterator
+        // because QHash::(const_)iterator does not work with size_t and thus will
+        // emit more warnings. Use the 4-arg std::is_permutation instead (which
+        // is supported since MSVC 2015).
+        //
+        // ### Qt 6: if C++14 library support is a mandated minimum, remove the ifdef for MSVC.
+        if (!std::is_permutation(it, thisEqualRangeEnd, otherEqualRange.first
+#if defined(Q_CC_MSVC) && _MSC_VER >= 1900
+                                 , otherEqualRange.second
+#endif
+                                 )) {
             return false;
+        }
 
         it = thisEqualRangeEnd;
     }