From 5dab710b90e7041aaa9fcf3631f2ca6af1baab5f Mon Sep 17 00:00:00 2001
From: Giuseppe D'Angelo <giuseppe.dangelo@kdab.com>
Date: Fri, 27 Nov 2020 17:31:03 +0100
Subject: [PATCH] Seedless qHash catch-all: make it SFINAE-friendly

To support qHash overloads without a seed we have a qHash(T, size_t)
catch-all that calls qHash(T) and XORs the seed. The problem is
that this catch-all is not SFINAE friendly. For a type Foo which
does not have any qHash overload, we can't ask if qHash(Foo, size_t)
is callable because it would instantiate the catch-all and fail
to compile.

Add a suitable trait and enable_if.

Pick-to: 6.0
Change-Id: Idffd48a537eebaf77cee7030b8d91a302643ffde
Reviewed-by: Fabian Kosmale <fabian.kosmale@qt.io>
Reviewed-by: Lars Knoll <lars.knoll@qt.io>
Reviewed-by: Andrei Golubev <andrei.golubev@qt.io>
---
 src/corelib/tools/qhashfunctions.h | 12 ++++++++++--
 1 file changed, 10 insertions(+), 2 deletions(-)

diff --git a/src/corelib/tools/qhashfunctions.h b/src/corelib/tools/qhashfunctions.h
index 9cfa852ba2..a03f659b52 100644
--- a/src/corelib/tools/qhashfunctions.h
+++ b/src/corelib/tools/qhashfunctions.h
@@ -90,6 +90,14 @@ Q_DECL_CONST_FUNCTION constexpr size_t hash(size_t key, size_t seed) noexcept
     }
 }
 
+template <typename T, typename = void>
+constexpr inline bool HasQHashSingleArgOverload = false;
+
+template <typename T>
+constexpr inline bool HasQHashSingleArgOverload<T, std::enable_if_t<
+    std::is_convertible_v<decltype(qHash(std::declval<const T &>())), size_t>
+>> = true;
+
 }
 
 Q_CORE_EXPORT Q_DECL_PURE_FUNCTION size_t qHashBits(const void *p, size_t size, size_t seed = 0) noexcept;
@@ -166,8 +174,8 @@ Q_DECL_CONST_FUNCTION constexpr inline size_t qHash(QKeyCombination key, size_t
 { return qHash(key.toCombined(), seed); }
 Q_CORE_EXPORT Q_DECL_PURE_FUNCTION uint qt_hash(QStringView key, uint chained = 0) noexcept;
 
-template<typename T> inline size_t qHash(const T &t, size_t seed)
-    noexcept(noexcept(qHash(t)))
+template <typename T, std::enable_if_t<QHashPrivate::HasQHashSingleArgOverload<T>, bool> = true>
+size_t qHash(const T &t, size_t seed) noexcept(noexcept(qHash(t)))
 { return qHash(t) ^ seed; }
 
 namespace QtPrivate {