Add qHash(std::pair)

We already include <utility> in <qglobal.h>, so we might
as well provide a qHash() overload for std::pair.

[ChangeLog][QtCore] Added qHash(std::pair), defined in
<QHashFunctions>.

Change-Id: I0f61c513e82e05ce9d2e56bcf18f3be9e2da4da9
Reviewed-by: Olivier Goffart (Woboq GmbH) <ogoffart@woboq.com>

src/corelib/tools/qhash.cpp

@@ -711,6 +711,23 @@ void QHashData::checkSanity()
     Types \c T1 and \c T2 must be supported by qHash().
 */
 
+/*!
+    \fn uint qHash(const std::pair<T1, T2> &key, uint seed = 0)
+    \since 5.7
+    \relates QHash
+
+    Returns the hash value for the \a key, using \a seed to seed the calculation.
+
+    Types \c T1 and \c T2 must be supported by qHash().
+
+    \note The return type of this function is \e{not} the same as that of
+    \code
+    qHash(qMakePair(key.first, key.second), seed);
+    \endcode
+    The two functions use different hashing algorithms; due to binary compatibility
+    constraints, we cannot change the QPair algorithm to match the std::pair one before Qt 6.
+*/
+
 /*! \fn uint qHashRange(InputIterator first, InputIterator last, uint seed = 0)
     \relates QHash
     \since 5.5

src/corelib/tools/qhashfunctions.h

@@ -147,6 +147,15 @@ template <typename T1, typename T2> inline uint qHash(const QPair<T1, T2> &key,
     return ((h1 << 16) | (h1 >> 16)) ^ h2 ^ seed;
 }
 
+template <typename T1, typename T2> inline uint qHash(const std::pair<T1, T2> &key, uint seed = 0)
+    Q_DECL_NOEXCEPT_EXPR(noexcept(qHash(key.first, seed)) && noexcept(qHash(key.second, seed)))
+{
+    QtPrivate::QHashCombine hash;
+    seed = hash(seed, key.first);
+    seed = hash(seed, key.second);
+    return seed;
+}
+
 QT_END_NAMESPACE
 
 #if defined(Q_CC_MSVC)

tests/auto/corelib/tools/qhashfunctions/tst_qhashfunctions.cpp

@@ -115,6 +115,22 @@ void tst_QHashFunctions::qhash()
 
         QVERIFY(qHash(pA) != qHash(pB));
     }
+
+    {
+        std::pair<int, int> p12(1, 2);
+        std::pair<int, int> p21(2, 1);
+
+        using QT_PREPEND_NAMESPACE(qHash);
+
+        QVERIFY(qHash(p12) == qHash(p12));
+        QVERIFY(qHash(p21) == qHash(p21));
+        QVERIFY(qHash(p12) != qHash(p21));
+
+        std::pair<int, int> pA(0x12345678, 0x12345678);
+        std::pair<int, int> pB(0x12345675, 0x12345675);
+
+        QVERIFY(qHash(pA) != qHash(pB));
+    }
 }
 
 void tst_QHashFunctions::fp_qhash_of_zero_is_zero()