QHash: fix qHash(std::pair)

There were two problems here: first, qHash(std::pair) must be declared
before qHashMulti that might call back to qHash(std::pair) (i.e., a pair
with one element that is also a pair). But moving the declaration above
causes the second problem: the noexcept expression can't refer to qHash
functions that aren't declared yet. So we forward-declare a constexpr
function for that result, but implement it far below.

Fixes: QTBUG-92910
Change-Id: Ia8e48103a54446509e3bfffd16767ed2e29b026c
Reviewed-by: Christian Kandeler <christian.kandeler@qt.io>
Reviewed-by: Thiago Macieira <thiago.macieira@intel.com>

2 files changed, 55 insertions, 2 deletions

diff --git a/src/corelib/tools/qhashfunctions.h b/src/corelib/tools/qhashfunctions.h
index 4c2eba3e53..4f8e4066ee 100644
--- a/src/corelib/tools/qhashfunctions.h
+++ b/src/corelib/tools/qhashfunctions.h
@@ -114,10 +114,15 @@ constexpr inline bool HasQHashSingleArgOverload<T, std::enable_if_t<
     std::is_convertible_v<decltype(qHash(std::declval<const T &>())), size_t>
 >> = true;
 
+template <typename T1, typename T2> static constexpr bool noexceptPairHash();
 }
 
 Q_CORE_EXPORT Q_DECL_PURE_FUNCTION size_t qHashBits(const void *p, size_t size, size_t seed = 0) noexcept;
 
+// implementation below qHashMulti
+template <typename T1, typename T2> inline size_t qHash(const std::pair<T1, T2> &key, size_t seed = 0)
+    noexcept(QHashPrivate::noexceptPairHash<T1, T2>());
+
 // C++ builtin types
 Q_DECL_CONST_FUNCTION constexpr inline size_t qHash(char key, size_t seed = 0) noexcept
 { return QHashPrivate::hash(size_t(key), seed); }
@@ -290,8 +295,16 @@ inline size_t qHashRangeCommutative(InputIterator first, InputIterator last, siz
     return std::accumulate(first, last, seed, QtPrivate::QHashCombineCommutative());
 }
 
-template <typename T1, typename T2> inline size_t qHash(const std::pair<T1, T2> &key, size_t seed = 0)
-    noexcept(noexcept(qHash(key.first, seed)) && noexcept(qHash(key.second, seed)))
+namespace QHashPrivate {
+template <typename T1, typename T2> static constexpr bool noexceptPairHash()
+{
+    size_t seed = 0;
+    return noexcept(qHash(std::declval<T1>(), seed)) && noexcept(qHash(std::declval<T2>(), seed));
+}
+} // QHashPrivate
+
+template <typename T1, typename T2> inline size_t qHash(const std::pair<T1, T2> &key, size_t seed)
+    noexcept(QHashPrivate::noexceptPairHash<T1, T2>())
 {
     return qHashMulti(seed, key.first, key.second);
 }
diff --git a/tests/auto/corelib/tools/qhashfunctions/tst_qhashfunctions.cpp b/tests/auto/corelib/tools/qhashfunctions/tst_qhashfunctions.cpp
index d313fbb0b0..a440d43454 100644
--- a/tests/auto/corelib/tools/qhashfunctions/tst_qhashfunctions.cpp
+++ b/tests/auto/corelib/tools/qhashfunctions/tst_qhashfunctions.cpp
@@ -46,6 +46,8 @@ public:
     };
     uint seed;
 
+    template <typename T1, typename T2> void stdPair_template(const T1 &t1, const T2 &t2);
+
 public slots:
     void initTestCase();
     void init();
@@ -63,6 +65,18 @@ private Q_SLOTS:
 
     void stdHash();
 
+    void stdPair_int_int()          { stdPair_template(1, 2); }
+    void stdPair_ulong_llong()      { stdPair_template(1UL, -2LL); }
+    void stdPair_ullong_long()      { stdPair_template(1ULL, -2L); }
+    void stdPair_string_int()       { stdPair_template(QString("Hello"), 2); }
+    void stdPair_int_string()       { stdPair_template(1, QString("Hello")); }
+    void stdPair_bytearray_string() { stdPair_template(QByteArray("Hello"), QString("World")); }
+    void stdPair_string_bytearray() { stdPair_template(QString("Hello"), QByteArray("World")); }
+    void stdPair_int_pairIntInt()   { stdPair_template(1, std::make_pair(2, 3)); }
+    void stdPair_2x_pairIntInt()    { stdPair_template(std::make_pair(1, 2), std::make_pair(2, 3)); }
+    void stdPair_string_pairIntInt()    { stdPair_template(QString("Hello"), std::make_pair(42, -47)); } // QTBUG-92910
+    void stdPair_int_pairIntPairIntInt() { stdPair_template(1, std::make_pair(2, std::make_pair(3, 4))); }
+
     void setGlobalQHashSeed();
 };
 
@@ -310,6 +324,32 @@ void tst_QHashFunctions::stdHash()
 
 }
 
+template <typename T1, typename T2>
+void tst_QHashFunctions::stdPair_template(const T1 &t1, const T2 &t2)
+{
+    std::pair<T1, T2> dpair{};
+    std::pair<T1, T2> vpair{t1, t2};
+
+    size_t seed = QHashSeed::globalSeed();
+
+    // confirm proper working of the pair and of the underlying types
+    QVERIFY(t1 == t1);
+    QVERIFY(t2 == t2);
+    QCOMPARE(qHash(t1), qHash(t1));
+    QCOMPARE(qHash(t2), qHash(t2));
+    QCOMPARE(qHash(t1, seed), qHash(t1, seed));
+    QCOMPARE(qHash(t2, seed), qHash(t2, seed));
+
+    QVERIFY(dpair == dpair);
+    QVERIFY(vpair == vpair);
+
+    // therefore their hashes should be equal
+    QCOMPARE(qHash(dpair), qHash(dpair));
+    QCOMPARE(qHash(dpair, seed), qHash(dpair, seed));
+    QCOMPARE(qHash(vpair), qHash(vpair));
+    QCOMPARE(qHash(vpair, seed), qHash(vpair, seed));
+}
+
 void tst_QHashFunctions::setGlobalQHashSeed()
 {
     // Setter works as advertised