Add missing <=> 0 operator to Qt ordering types

It's required by the standard, see e.g. [1] and the eel.is links in
the code.

[ChangeLog][QtCore][QPartialOrdering] Added three-way comparison
operator (<=>) against literal zero, available when compiling in C++20
mode.

[1] https://en.cppreference.com/w/cpp/utility/compare/partial_ordering#Comparisons

Change-Id: I8a3b76661400930c6e247cf5b138ff52bf784395
Reviewed-by: Qt CI Bot <qt_ci_bot@qt-project.org>
Reviewed-by: Ivan Solovev <ivan.solovev@qt.io>

2 files changed, 62 insertions, 0 deletions

diff --git a/src/corelib/global/qcompare.h b/src/corelib/global/qcompare.h
index 35123dcb36..9e67bdaa94 100644
--- a/src/corelib/global/qcompare.h
+++ b/src/corelib/global/qcompare.h
@@ -47,6 +47,19 @@ enum class Uncomparable : CompareUnderlyingType
 
 } // namespace QtPrivate
 
+namespace QtOrderingPrivate {
+
+template <typename O>
+constexpr O reversed(O o) noexcept
+{
+    // https://eel.is/c++draft/cmp.partialord#5
+    return is_lt(o) ? O::greater :
+           is_gt(o) ? O::less :
+           /*else*/ o ;
+}
+
+} // namespace QtOrderingPrivate
+
 namespace Qt {
 
 class partial_ordering
@@ -107,6 +120,17 @@ public:
     { return rhs.isOrdered() && 0 >= rhs.m_order; }
 
 
+#ifdef __cpp_lib_three_way_comparison
+    friend constexpr std::partial_ordering
+    operator<=>(partial_ordering lhs, QtPrivate::CompareAgainstLiteralZero) noexcept
+    { return lhs; } // https://eel.is/c++draft/cmp.partialord#4
+
+    friend constexpr std::partial_ordering
+    operator<=>(QtPrivate::CompareAgainstLiteralZero, partial_ordering rhs) noexcept
+    { return QtOrderingPrivate::reversed(rhs); }
+#endif // __cpp_lib_three_way_comparison
+
+
     friend constexpr bool operator==(partial_ordering lhs, partial_ordering rhs) noexcept
     { return lhs.m_order == rhs.m_order; }
 
@@ -253,6 +277,17 @@ public:
     { return 0 >= rhs.m_order; }
 
 
+#ifdef __cpp_lib_three_way_comparison
+    friend constexpr std::weak_ordering
+    operator<=>(weak_ordering lhs, QtPrivate::CompareAgainstLiteralZero) noexcept
+    { return lhs; } // https://eel.is/c++draft/cmp.weakord#5
+
+    friend constexpr std::weak_ordering
+    operator<=>(QtPrivate::CompareAgainstLiteralZero, weak_ordering rhs) noexcept
+    { return QtOrderingPrivate::reversed(rhs); }
+#endif // __cpp_lib_three_way_comparison
+
+
     friend constexpr bool operator==(weak_ordering lhs, weak_ordering rhs) noexcept
     { return lhs.m_order == rhs.m_order; }
 
@@ -425,6 +460,17 @@ public:
     { return 0 >= rhs.m_order; }
 
 
+#ifdef __cpp_lib_three_way_comparison
+    friend constexpr std::strong_ordering
+    operator<=>(strong_ordering lhs, QtPrivate::CompareAgainstLiteralZero) noexcept
+    { return lhs; } // https://eel.is/c++draft/cmp.strongord#6
+
+    friend constexpr std::strong_ordering
+    operator<=>(QtPrivate::CompareAgainstLiteralZero, strong_ordering rhs) noexcept
+    { return QtOrderingPrivate::reversed(rhs); }
+#endif // __cpp_lib_three_way_comparison
+
+
     friend constexpr bool operator==(strong_ordering lhs, strong_ordering rhs) noexcept
     { return lhs.m_order == rhs.m_order; }
 
@@ -699,6 +745,17 @@ public:
     { return rhs.isOrdered() && 0 >= rhs.m_order; }
 
 
+#ifdef __cpp_lib_three_way_comparison
+    friend constexpr std::partial_ordering
+    operator<=>(QPartialOrdering lhs, QtPrivate::CompareAgainstLiteralZero) noexcept
+    { return lhs; } // https://eel.is/c++draft/cmp.partialord#4
+
+    friend constexpr std::partial_ordering
+    operator<=>(QtPrivate::CompareAgainstLiteralZero, QPartialOrdering rhs) noexcept
+    { return QtOrderingPrivate::reversed(rhs); }
+#endif // __cpp_lib_three_way_comparison
+
+
     friend constexpr bool operator==(QPartialOrdering lhs, QPartialOrdering rhs) noexcept
     { return lhs.m_order == rhs.m_order; }
 
diff --git a/src/corelib/global/qcomparehelpers.h b/src/corelib/global/qcomparehelpers.h
index d7e007b8b0..0394703e17 100644
--- a/src/corelib/global/qcomparehelpers.h
+++ b/src/corelib/global/qcomparehelpers.h
@@ -27,6 +27,8 @@
 
 QT_BEGIN_NAMESPACE
 
+class QPartialOrdering;
+
 namespace QtOrderingPrivate {
 #ifdef __cpp_lib_three_way_comparison
 
@@ -44,6 +46,9 @@ QT_STD_MAP(weak)
 QT_STD_MAP(strong)
 #undef QT_STD_MAP
 
+template <> struct StdOrdering<QPartialOrdering> : q20::type_identity<std::partial_ordering> {};
+template <> struct  QtOrdering<QPartialOrdering> : q20::type_identity< Qt::partial_ordering> {};
+
 template <typename In> constexpr auto to_std(In in) noexcept
     -> typename QtOrderingPrivate::StdOrdering<In>::type
 { return in; }