Centralize the MSVC work-around for std::is_permutation

There's currently only one user, but another one is coming up, so apply DRY
and centralize the work-around for the MSVC warning C4996 on use of 3-arg STL
algorithms in one place.

The code is prepared to handle other algorithms with ease, should any more
crop up.

Change-Id: Ia881888d6a2b5286c6d8d823bc2b76788efad624
Reviewed-by: Thiago Macieira <thiago.macieira@intel.com>

2 files changed, 24 insertions, 15 deletions

diff --git a/src/corelib/global/qglobal.h b/src/corelib/global/qglobal.h
index 15f4621cc0..80f59d92d0 100644
--- a/src/corelib/global/qglobal.h
+++ b/src/corelib/global/qglobal.h
@@ -1001,6 +1001,29 @@ QT_WARNING_DISABLE_MSVC(4530) /* C++ exception handler used, but unwind semantic
 #  endif
 #endif
 
+// Work around MSVC warning about use of 3-arg algorithms
+// until we can depend on the C++14 4-arg ones.
+//
+// These algortithms do NOT check for equal length.
+// They need to be treated as if they called the 3-arg version (which they do)!
+#ifdef Q_CC_MSVC
+# define QT_3ARG_ALG(alg, f1, l1, f2, l2) \
+    std::alg(f1, l1, f2, l2)
+#else
+# define QT_3ARG_ALG(alg, f1, l1, f2, l2)     \
+    [&f1, &l1, &f2, &l2]() {                  \
+        Q_UNUSED(l2);                         \
+        return std::alg(f1, l1, f2);          \
+    }()
+#endif
+template <typename ForwardIterator1, typename ForwardIterator2>
+inline bool qt_is_permutation(ForwardIterator1 first1, ForwardIterator1 last1,
+                              ForwardIterator2 first2, ForwardIterator2 last2)
+{
+    return QT_3ARG_ALG(is_permutation, first1, last1, first2, last2);
+}
+#undef QT_3ARG_ALG
+
 // this adds const to non-const objects (like std::as_const)
 template <typename T>
 Q_DECL_CONSTEXPR typename std::add_const<T>::type &qAsConst(T &t) noexcept { return t; }
diff --git a/src/corelib/tools/qhash.h b/src/corelib/tools/qhash.h
index ee754a79d5..236e433101 100644
--- a/src/corelib/tools/qhash.h
+++ b/src/corelib/tools/qhash.h
@@ -981,22 +981,8 @@ 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.
-        //
-        // 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(thisEqualRangeStart, it, otherEqualRange.first
-#ifdef Q_CC_MSVC
-                                 , otherEqualRange.second
-#endif
-                                 )) {
+        if (!qt_is_permutation(thisEqualRangeStart, it, otherEqualRange.first, otherEqualRange.second))
             return false;
-        }
     }
 
     return true;