Make QHashDummyValue more robust

As is customary for tag structs, make the default ctor explicit to
avoid construction as '{}'.

Make op== constexpr (drive-by: make it a hidden friend instead of a
member function), and add op!= to C++17 builds, for symmetry.

=delete the qHash() overload so it doesn't accidentally match
some other overload.

We should eventually also =delete equality, but atm, this breaks
QSet::compareEquals, which calls QHash::op==, which (incorrectly)
SFINAE's out for a non-equality-comparable QHashDummyValue
mapped_type.

Amends 5b7c3e31b538376f2b4733bd868b5875b504cdb3.

Not picking back, as it's a rather isolated change with very little
chance of creating conflicts going forward, but non-negligible chance
of breaking something in older branches.

Change-Id: I8125581c476f854ebe4f9ddc791d3ddce9f169d0
Reviewed-by: Thiago Macieira <[email protected]>

1 files changed, 7 insertions, 1 deletions

diff --git a/src/corelib/tools/qhash.h b/src/corelib/tools/qhash.h
index 76729917f6b..6509048da0c 100644
--- a/src/corelib/tools/qhash.h
+++ b/src/corelib/tools/qhash.h
@@ -23,7 +23,13 @@ QT_BEGIN_NAMESPACE
 
 struct QHashDummyValue
 {
-    bool operator==(const QHashDummyValue &) const noexcept { return true; }
+    explicit QHashDummyValue() = default;
+    friend constexpr bool operator==(QHashDummyValue, QHashDummyValue) noexcept { return true; }
+#ifndef __cpp_impl_three_way_comparison
+    friend constexpr bool operator!=(QHashDummyValue, QHashDummyValue) noexcept { return false; }
+#endif
+    friend constexpr size_t qHash(QHashDummyValue) noexcept = delete;
+    friend constexpr size_t qHash(QHashDummyValue, size_t) noexcept = delete;
 };
 
 namespace QHashPrivate {