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>

1 files changed, 15 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);
 }