Fix QVariant/QMetaType::compare APIs

std::optional<int> is the wrong datatype to use for compare.

First and foremost, it can't be used in the idiomatic form of

  auto r = a.compare(b);
  if (r < 0) ~~~ // a is less than b
  if (r > 0) ~~~ // a is greater than b

which we *already* feature in Qt (QString, QByteArray).

Also, std::optional<int> (explicitly) converts to bool, which is
a trap, because the result of the comparison can be accidentally
tested as a bool:

  if (a.compare(b)) ~~~ // oops! does NOT mean a<b

Not to mention extending this to algorithms:

  auto lessThan = [](QVariant a, QVariant b) { return a.compare(b); }; // oops!
  std::ranges::sort(vectorOfVariants, lessThan);

which thankfully doesn't compile as is -- std::optional has
an *explicit* operator bool, and the Compare concept requires an
implicit conversion. However, the error the user is going to face
will be "cannot convert to bool because the operator is explicit",
which is deceiving because the fix is NOT supposed to be:

  auto lessThan = [](QVariant a, QVariant b) { return (bool)a.compare(b); }; // big oops!

Instead: backport to Qt the required subset of C++20's <compare>
API, and use that. This commits just adds the necessary parts
for compare() (i.e. partial ordering), the rest of <compare>
(classes, functions, conversions) can be added to 6.1.

Change-Id: I2b5522da47854da39f79993e1207fad033786f00
Reviewed-by: Volker Hilsheimer <volker.hilsheimer@qt.io>
(cherry picked from commit 3e59c97c3453926fc66479d9ceca03901df55f90)
Reviewed-by: Qt Cherry-pick Bot <cherrypick_bot@qt-project.org>

14 files changed, 511 insertions, 29 deletions

diff --git a/src/corelib/CMakeLists.txt b/src/corelib/CMakeLists.txt
index 9aad3c4980..c14bcf91bb 100644
--- a/src/corelib/CMakeLists.txt
+++ b/src/corelib/CMakeLists.txt
@@ -32,6 +32,7 @@ qt_internal_add_module(Core
     EXCEPTIONS
     SOURCES
         global/archdetect.cpp
+        global/qcompare.h
         global/qcompilerdetection.h
         global/qcontainerinfo.h
         global/qendian.cpp global/qendian.h global/qendian_p.h
diff --git a/src/corelib/global/global.pri b/src/corelib/global/global.pri
index 5850792bdb..14067f638a 100644
--- a/src/corelib/global/global.pri
+++ b/src/corelib/global/global.pri
@@ -5,6 +5,7 @@ HEADERS +=  \
         global/qoperatingsystemversion.h \
         global/qoperatingsystemversion_p.h \
         global/qsystemdetection.h \
+        global/qcompare.h \
         global/qcompilerdetection.h \
         global/qcontainerinfo.h \
         global/qprocessordetection.h \
diff --git a/src/corelib/global/qcompare.h b/src/corelib/global/qcompare.h
new file mode 100644
index 0000000000..07f4589984
--- /dev/null
+++ b/src/corelib/global/qcompare.h
@@ -0,0 +1,143 @@
+/****************************************************************************
+**
+** Copyright (C) 2020 Klarälvdalens Datakonsult AB, a KDAB Group company, info@kdab.com, author Giuseppe D'Angelo <giuseppe.dangelo@kdab.com>
+** Contact: http://www.qt.io/licensing/
+**
+** This file is part of the QtCore module of the Qt Toolkit.
+**
+** $QT_BEGIN_LICENSE:LGPL$
+** Commercial License Usage
+** Licensees holding valid commercial Qt licenses may use this file in
+** accordance with the commercial license agreement provided with the
+** Software or, alternatively, in accordance with the terms contained in
+** a written agreement between you and The Qt Company. For licensing terms
+** and conditions see https://www.qt.io/terms-conditions. For further
+** information use the contact form at https://www.qt.io/contact-us.
+**
+** GNU Lesser General Public License Usage
+** Alternatively, this file may be used under the terms of the GNU Lesser
+** General Public License version 3 as published by the Free Software
+** Foundation and appearing in the file LICENSE.LGPL3 included in the
+** packaging of this file. Please review the following information to
+** ensure the GNU Lesser General Public License version 3 requirements
+** will be met: https://www.gnu.org/licenses/lgpl-3.0.html.
+**
+** GNU General Public License Usage
+** Alternatively, this file may be used under the terms of the GNU
+** General Public License version 2.0 or (at your option) the GNU General
+** Public license version 3 or any later version approved by the KDE Free
+** Qt Foundation. The licenses are as published by the Free Software
+** Foundation and appearing in the file LICENSE.GPL2 and LICENSE.GPL3
+** included in the packaging of this file. Please review the following
+** information to ensure the GNU General Public License requirements will
+** be met: https://www.gnu.org/licenses/gpl-2.0.html and
+** https://www.gnu.org/licenses/gpl-3.0.html.
+**
+** $QT_END_LICENSE$
+**
+****************************************************************************/
+
+#ifndef QCOMPARE_H
+#define QCOMPARE_H
+
+#if 0
+#pragma qt_class(QtCompare)
+#endif
+
+#include <QtCore/qglobal.h>
+
+QT_BEGIN_NAMESPACE
+
+namespace QtPrivate {
+using CompareUnderlyingType = qint8;
+
+// [cmp.categories.pre] / 1
+enum class Ordering : CompareUnderlyingType
+{
+    Equal = 0,
+    Equivalent = Equal,
+    Less = -1,
+    Greater = 1
+};
+
+enum class Uncomparable : CompareUnderlyingType
+{
+    Unordered = -127
+};
+
+// [cmp.categories.pre] / 3, but using a safe bool trick
+// and also rejecting std::nullptr_t (unlike the example)
+class CompareAgainstLiteralZero {
+public:
+    using SafeZero = void (CompareAgainstLiteralZero::*)();
+    Q_IMPLICIT constexpr CompareAgainstLiteralZero(SafeZero) noexcept {}
+
+    template <typename T, std::enable_if_t<!std::is_same_v<T, int>, bool> = false>
+    CompareAgainstLiteralZero(T) = delete;
+};
+} // namespace QtPrivate
+
+// [cmp.partialord]
+class QPartialOrdering
+{
+public:
+    static const QPartialOrdering Less;
+    static const QPartialOrdering Equivalent;
+    static const QPartialOrdering Greater;
+    static const QPartialOrdering Unordered;
+
+    friend constexpr bool operator==(QPartialOrdering p, QtPrivate::CompareAgainstLiteralZero) noexcept
+    { return p.isOrdered() && p.m_order == 0; }
+    friend constexpr bool operator!=(QPartialOrdering p, QtPrivate::CompareAgainstLiteralZero) noexcept
+    { return p.isOrdered() && p.m_order != 0; }
+    friend constexpr bool operator< (QPartialOrdering p, QtPrivate::CompareAgainstLiteralZero) noexcept
+    { return p.isOrdered() && p.m_order <  0; }
+    friend constexpr bool operator<=(QPartialOrdering p, QtPrivate::CompareAgainstLiteralZero) noexcept
+    { return p.isOrdered() && p.m_order <= 0; }
+    friend constexpr bool operator> (QPartialOrdering p, QtPrivate::CompareAgainstLiteralZero) noexcept
+    { return p.isOrdered() && p.m_order >  0; }
+    friend constexpr bool operator>=(QPartialOrdering p, QtPrivate::CompareAgainstLiteralZero) noexcept
+    { return p.isOrdered() && p.m_order >= 0; }
+
+    friend constexpr bool operator==(QtPrivate::CompareAgainstLiteralZero, QPartialOrdering p) noexcept
+    { return p.isOrdered() && 0 == p.m_order; }
+    friend constexpr bool operator!=(QtPrivate::CompareAgainstLiteralZero, QPartialOrdering p) noexcept
+    { return p.isOrdered() && 0 != p.m_order; }
+    friend constexpr bool operator< (QtPrivate::CompareAgainstLiteralZero, QPartialOrdering p) noexcept
+    { return p.isOrdered() && 0 <  p.m_order; }
+    friend constexpr bool operator<=(QtPrivate::CompareAgainstLiteralZero, QPartialOrdering p) noexcept
+    { return p.isOrdered() && 0 <= p.m_order; }
+    friend constexpr bool operator> (QtPrivate::CompareAgainstLiteralZero, QPartialOrdering p) noexcept
+    { return p.isOrdered() && 0 >  p.m_order; }
+    friend constexpr bool operator>=(QtPrivate::CompareAgainstLiteralZero, QPartialOrdering p) noexcept
+    { return p.isOrdered() && 0 >= p.m_order; }
+
+    friend constexpr bool operator==(QPartialOrdering p1, QPartialOrdering p2) noexcept
+    { return p1.m_order == p2.m_order; }
+    friend constexpr bool operator!=(QPartialOrdering p1, QPartialOrdering p2) noexcept
+    { return p1.m_order != p2.m_order; }
+
+private:
+    constexpr explicit QPartialOrdering(QtPrivate::Ordering order) noexcept
+        : m_order(static_cast<QtPrivate::CompareUnderlyingType>(order))
+    {}
+    constexpr explicit QPartialOrdering(QtPrivate::Uncomparable order) noexcept
+        : m_order(static_cast<QtPrivate::CompareUnderlyingType>(order))
+    {}
+
+    // instead of the exposition only is_ordered member in [cmp.partialord],
+    // use a private function
+    constexpr bool isOrdered() noexcept
+    { return m_order != static_cast<QtPrivate::CompareUnderlyingType>(QtPrivate::Uncomparable::Unordered); }
+
+    QtPrivate::CompareUnderlyingType m_order;
+};
+
+inline constexpr QPartialOrdering QPartialOrdering::Less(QtPrivate::Ordering::Less);
+inline constexpr QPartialOrdering QPartialOrdering::Equivalent(QtPrivate::Ordering::Equivalent);
+inline constexpr QPartialOrdering QPartialOrdering::Greater(QtPrivate::Ordering::Greater);
+inline constexpr QPartialOrdering QPartialOrdering::Unordered(QtPrivate::Uncomparable::Unordered);
+
+QT_END_NAMESPACE
+
+#endif // QCOMPARE_H
diff --git a/src/corelib/global/qcompare.qdoc b/src/corelib/global/qcompare.qdoc
new file mode 100644
index 0000000000..80d402f67c
--- /dev/null
+++ b/src/corelib/global/qcompare.qdoc
@@ -0,0 +1,147 @@
+/****************************************************************************
+**
+** Copyright (C) 2020 Klarälvdalens Datakonsult AB, a KDAB Group company, info@kdab.com, author Giuseppe D'Angelo <giuseppe.dangelo@kdab.com>
+** Contact: http://www.qt.io/licensing/
+**
+** This file is part of the documentation of the Qt Toolkit.
+**
+** $QT_BEGIN_LICENSE:FDL$
+** Commercial License Usage
+** Licensees holding valid commercial Qt licenses may use this file in
+** accordance with the commercial license agreement provided with the
+** Software or, alternatively, in accordance with the terms contained in
+** a written agreement between you and The Qt Company. For licensing terms
+** and conditions see https://www.qt.io/terms-conditions. For further
+** information use the contact form at https://www.qt.io/contact-us.
+**
+** GNU Free Documentation License Usage
+** Alternatively, this file may be used under the terms of the GNU Free
+** Documentation License version 1.3 as published by the Free Software
+** Foundation and appearing in the file included in the packaging of
+** this file. Please review the following information to ensure
+** the GNU Free Documentation License version 1.3 requirements
+** will be met: https://www.gnu.org/licenses/fdl-1.3.html.
+** $QT_END_LICENSE$
+**
+****************************************************************************/
+
+/*!
+    \class QPartialOrdering
+    \inmodule QtCore
+    \brief QPartialOrdering represents the result of a comparison that allows for unordered results.
+    \since 6.0
+
+    A value of type QPartialOrdering is typically returned from a
+    three-way comparison function. Such a function compares two
+    objects, and it may either establish that the two objects are
+    ordered relative to each other, or that they are not ordered. The
+    QPartialOrdering value returned from the comparison function
+    represents one of those possibilities.
+
+    The possible values of type QPartialOrdering are, in fact, fully
+    represented by the following four static values:
+
+    \list
+
+    \li \c QPartialOrdering::Less represents that the first object is
+    less than the second;
+
+    \li \c QPartialOrdering::Equivalent represents that the first
+    object is equivalent to the second;
+
+    \li \c QPartialOrdering::Greater represents that the first object
+    is equivalent to the second;
+
+    \li \c QPartialOrdering::Unordered represents that the first object
+    is \e{not ordered} with respect to the second.
+
+    \endlist
+
+    QPartialOrdering is idiomatically used by comparing an instance
+    against a literal zero, for instance like this:
+
+    \code
+
+    // given a, b, c, d as objects of some type that allows for a 3-way compare
+
+    QPartialOrdering result = a.compare(b);
+    if (result < 0) {
+        // a is less than b
+    }
+
+    if (c.compare(d) >= 0) {
+        // c is greater than or equal to d
+    }
+
+    \endcode
+
+    A QPartialOrdering value which represents an unordered result will
+    always return false when compared against literal 0.
+*/
+
+/*!
+    \fn bool operator==(QPartialOrdering p1, QPartialOrdering p2) noexcept
+    \relates QPartialOrdering
+
+    Return true if \a p1 and \a p2 represent the same result;
+    otherwise, returns false.
+*/
+
+/*!
+    \fn bool operator!=(QPartialOrdering p1, QPartialOrdering p2) noexcept
+    \relates QPartialOrdering
+
+    Return true if \a p1 and \a p2 represent different results;
+    otherwise, returns true.
+*/
+
+/*!
+    \fn bool operator==(QPartialOrdering p, QtPrivate::CompareAgainstLiteralZero) noexcept
+    \fn bool operator!=(QPartialOrdering p, QtPrivate::CompareAgainstLiteralZero) noexcept
+    \fn bool operator< (QPartialOrdering p, QtPrivate::CompareAgainstLiteralZero) noexcept
+    \fn bool operator<=(QPartialOrdering p, QtPrivate::CompareAgainstLiteralZero) noexcept
+    \fn bool operator> (QPartialOrdering p, QtPrivate::CompareAgainstLiteralZero) noexcept
+    \fn bool operator>=(QPartialOrdering p, QtPrivate::CompareAgainstLiteralZero) noexcept
+
+    \fn bool operator==(QtPrivate::CompareAgainstLiteralZero, QPartialOrdering p) noexcept
+    \fn bool operator!=(QtPrivate::CompareAgainstLiteralZero, QPartialOrdering p) noexcept
+    \fn bool operator< (QtPrivate::CompareAgainstLiteralZero, QPartialOrdering p) noexcept
+    \fn bool operator<=(QtPrivate::CompareAgainstLiteralZero, QPartialOrdering p) noexcept
+    \fn bool operator> (QtPrivate::CompareAgainstLiteralZero, QPartialOrdering p) noexcept
+    \fn bool operator>=(QtPrivate::CompareAgainstLiteralZero, QPartialOrdering p) noexcept
+    \relates QPartialOrdering
+    \internal
+*/
+
+/*!
+    \value QPartialOrdering::Less
+    \relates QPartialOrdering
+
+    Represents the result of a comparison where the value on the left
+    hand side is less than the value on right hand side.
+*/
+
+/*!
+    \value QPartialOrdering::Equivalent
+    \relates QPartialOrdering
+
+    Represents the result of a comparison where the value on the left
+    hand side is equivalent to the value on right hand side.
+*/
+
+/*!
+    \value QPartialOrdering::Greater
+    \relates QPartialOrdering
+
+    Represents the result of a comparison where the value on the left
+    hand side is greater than the value on right hand side.
+*/
+
+/*!
+    \value QPartialOrdering::Unordered
+    \relates QPartialOrdering
+
+    Represents the result of a comparison where the value on the left
+    hand side is not ordered with respect to the value on right hand
+    side.
+*/
diff --git a/src/corelib/kernel/qmetatype.cpp b/src/corelib/kernel/qmetatype.cpp
index ab3e47d4b1..1681e22fba 100644
--- a/src/corelib/kernel/qmetatype.cpp
+++ b/src/corelib/kernel/qmetatype.cpp
@@ -666,12 +666,24 @@ void QMetaType::destruct(void *data) const
     }
 }
 
+static QPartialOrdering threeWayCompare(const void *ptr1, const void *ptr2)
+{
+    std::less<const void *> less;
+    if (less(ptr1, ptr2))
+        return QPartialOrdering::Less;
+    if (less(ptr2, ptr1))
+        return QPartialOrdering::Greater;
+    return QPartialOrdering::Equivalent;
+}
+
 /*!
     Compares the objects at \a lhs and \a rhs for ordering.
 
-    Returns an empty optional if comparison is not supported or the values are unordered.
-    Otherwise, returns -1, 0 or +1 according as \a lhs is less than, equal to or greater
-    than \a rhs.
+    Returns QPartialOrdering::Unordered if comparison is not supported
+    or the values are unordered. Otherwise, returns
+    QPartialOrdering::Less, QPartialOrdering::Equivalent or
+    QPartialOrdering::Greater if \a lhs is less than, equivalent
+    to or greater than \a rhs, respectively.
 
     Both objects must be of the type described by this metatype. If either \a lhs
     or \a rhs is \nullptr, the values are unordered. Comparison is only supported
@@ -690,24 +702,24 @@ void QMetaType::destruct(void *data) const
     \since 6.0
     \sa equals(), isOrdered()
 */
-std::optional<int> QMetaType::compare(const void *lhs, const void *rhs) const
+QPartialOrdering QMetaType::compare(const void *lhs, const void *rhs) const
 {
     if (!lhs || !rhs)
-        return std::optional<int>{};
+        return QPartialOrdering::Unordered;
     if (d_ptr->flags & QMetaType::IsPointer)
-        return std::less<const void *>()(*reinterpret_cast<const void * const *>(lhs),
-                                         *reinterpret_cast<const void * const *>(rhs));
+        return threeWayCompare(*reinterpret_cast<const void * const *>(lhs),
+                               *reinterpret_cast<const void * const *>(rhs));
     if (d_ptr && d_ptr->lessThan) {
         if (d_ptr->equals && d_ptr->equals(d_ptr, lhs, rhs))
-            return 0;
+            return QPartialOrdering::Equivalent;
         if (d_ptr->lessThan(d_ptr, lhs, rhs))
-            return -1;
+            return QPartialOrdering::Less;
         if (d_ptr->lessThan(d_ptr, rhs, lhs))
-            return 1;
+            return QPartialOrdering::Greater;
         if (!d_ptr->equals)
-            return 0;
+            return QPartialOrdering::Equivalent;
     }
-    return std::optional<int>{};
+    return QPartialOrdering::Unordered;
 }
 
 /*!
diff --git a/src/corelib/kernel/qmetatype.h b/src/corelib/kernel/qmetatype.h
index de6632ffc6..7f61e28c37 100644
--- a/src/corelib/kernel/qmetatype.h
+++ b/src/corelib/kernel/qmetatype.h
@@ -45,6 +45,7 @@
 #include <QtCore/qglobal.h>
 #include <QtCore/qatomic.h>
 #include <QtCore/qbytearray.h>
+#include <QtCore/qcompare.h>
 #include <QtCore/qvarlengtharray.h>
 #include <QtCore/qrefcount.h>
 #include <QtCore/qdatastream.h>
@@ -58,7 +59,6 @@
 #include <vector>
 #include <list>
 #include <map>
-#include <optional>
 #include <functional>
 
 #ifdef Bool
@@ -419,7 +419,7 @@ public:
     void destroy(void *data) const;
     void *construct(void *where, const void *copy = nullptr) const;
     void destruct(void *data) const;
-    std::optional<int> compare(const void *lhs, const void *rhs) const;
+    QPartialOrdering compare(const void *lhs, const void *rhs) const;
     bool equals(const void *lhs, const void *rhs) const;
 
     bool isEqualityComparable() const;
@@ -602,12 +602,19 @@ public:
     {
         QMetaType t(typeId);
         auto c = t.compare(lhs, rhs);
-        if (!c) {
+        if (c == QPartialOrdering::Unordered) {
             *result = 0;
             return false;
+        } else if (c == QPartialOrdering::Less) {
+            *result = -1;
+            return true;
+        } else if (c == QPartialOrdering::Equivalent) {
+            *result = 0;
+            return true;
+        } else {
+            *result = 1;
+            return true;
         }
-        *result = *c;
-        return true;
     }
     QT_DEPRECATED_VERSION_6_0
     static bool equals(const void *lhs, const void *rhs, int typeId, int *result)
diff --git a/src/corelib/kernel/qvariant.cpp b/src/corelib/kernel/qvariant.cpp
index 8d4bb38dc6..0f92307d8c 100644
--- a/src/corelib/kernel/qvariant.cpp
+++ b/src/corelib/kernel/qvariant.cpp
@@ -2352,17 +2352,31 @@ bool QVariant::equals(const QVariant &v) const
     return metatype.equals(d.storage(), v.d.storage());
 }
 
+static QPartialOrdering convertOptionalToPartialOrdering(const std::optional<int> &opt)
+{
+    if (!opt)
+        return QPartialOrdering::Unordered;
+    else if (*opt < 0)
+        return QPartialOrdering::Less;
+    else if (*opt == 0)
+        return QPartialOrdering::Equivalent;
+    else
+        return QPartialOrdering::Greater;
+}
+
 /*!
     Compares the objects at \a lhs and \a rhs for ordering.
 
-    Returns an std::nullopt if comparison is not supported or the values are unordered.
-    Otherwise, returns -1, 0 or +1 according as \a lhs is less than, equal to or greater
-    than \a rhs.
+    Returns QPartialOrdering::Unordered if comparison is not supported
+    or the values are unordered. Otherwise, returns
+    QPartialOrdering::Less, QPartialOrdering::Equivalent or
+    QPartialOrdering::Greater if \a lhs is less than, equivalent
+    to or greater than \a rhs, respectively.
 
     If the variants contain data with a different metatype, the values are considered
     unordered unless they are both of numeric or pointer types, where regular numeric or
     pointer comparison rules will be used.
-    \note: If a numeric comparison is done and at least one value is NaN, \c std::nullopt
+    \note: If a numeric comparison is done and at least one value is NaN, QPartialOrdering::Unordered
     is returned.
 
     If both variants contain data of the same metatype, the method will use the
@@ -2372,18 +2386,18 @@ bool QVariant::equals(const QVariant &v) const
     \since 6.0
     \sa QMetaType::compare(), QMetaType::isOrdered()
 */
-std::optional<int> QVariant::compare(const QVariant &lhs, const QVariant &rhs)
+QPartialOrdering QVariant::compare(const QVariant &lhs, const QVariant &rhs)
 {
     QMetaType t = lhs.d.type();
     if (t != rhs.d.type()) {
         // try numeric comparisons, with C++ type promotion rules (no conversion)
         if (qIsNumericType(lhs.d.typeId()) && qIsNumericType(rhs.d.typeId()))
-            return numericCompare(&lhs.d, &rhs.d);
+            return convertOptionalToPartialOrdering(numericCompare(&lhs.d, &rhs.d));
 #ifndef QT_BOOTSTRAPPED
         if (canConvertMetaObject(lhs.metaType(), rhs.metaType()))
-            return pointerCompare(&lhs.d, &rhs.d);
+            return convertOptionalToPartialOrdering(pointerCompare(&lhs.d, &rhs.d));
 #endif
-        return std::nullopt;
+        return QPartialOrdering::Unordered;
     }
     return t.compare(lhs.constData(), rhs.constData());
 }
diff --git a/src/corelib/kernel/qvariant.h b/src/corelib/kernel/qvariant.h
index 810f4aac89..8c3a27e3ca 100644
--- a/src/corelib/kernel/qvariant.h
+++ b/src/corelib/kernel/qvariant.h
@@ -507,7 +507,7 @@ public:
         }
     };
  public:
-    static std::optional<int> compare(const QVariant &lhs, const QVariant &rhs);
+    static QPartialOrdering compare(const QVariant &lhs, const QVariant &rhs);
 
 private:
     friend inline bool operator==(const QVariant &a, const QVariant &b)
diff --git a/tests/auto/corelib/global/CMakeLists.txt b/tests/auto/corelib/global/CMakeLists.txt
index 91890462fe..8ba71543c2 100644
--- a/tests/auto/corelib/global/CMakeLists.txt
+++ b/tests/auto/corelib/global/CMakeLists.txt
@@ -1,5 +1,6 @@
 # Generated from global.pro.
 
+add_subdirectory(qcompare)
 add_subdirectory(qflags)
 add_subdirectory(q_func_info)
 add_subdirectory(qgetputenv)
diff --git a/tests/auto/corelib/global/global.pro b/tests/auto/corelib/global/global.pro
index 80ac1a3f83..b3f3be08a5 100644
--- a/tests/auto/corelib/global/global.pro
+++ b/tests/auto/corelib/global/global.pro
@@ -1,5 +1,6 @@
 TEMPLATE=subdirs
 SUBDIRS=\
+    qcompare \
     qflags \
     q_func_info \
     qgetputenv \
diff --git a/tests/auto/corelib/global/qcompare/CMakeLists.txt b/tests/auto/corelib/global/qcompare/CMakeLists.txt
new file mode 100644
index 0000000000..0fbf696d0e
--- /dev/null
+++ b/tests/auto/corelib/global/qcompare/CMakeLists.txt
@@ -0,0 +1,13 @@
+# Generated from qcompare.pro.
+
+#####################################################################
+## tst_qcompare Test:
+#####################################################################
+
+qt_internal_add_test(tst_qcompare
+    SOURCES
+        tst_qcompare.cpp
+)
+
+## Scopes:
+#####################################################################
diff --git a/tests/auto/corelib/global/qcompare/qcompare.pro b/tests/auto/corelib/global/qcompare/qcompare.pro
new file mode 100644
index 0000000000..0e2cd2771c
--- /dev/null
+++ b/tests/auto/corelib/global/qcompare/qcompare.pro
@@ -0,0 +1,8 @@
+CONFIG += testcase
+TARGET = tst_qcompare
+QT = core testlib
+SOURCES = tst_qcompare.cpp
+qtConfig(c++11): CONFIG += c++11
+qtConfig(c++14): CONFIG += c++14
+qtConfig(c++17): CONFIG += c++17
+qtConfig(c++2a): CONFIG += c++2a
diff --git a/tests/auto/corelib/global/qcompare/tst_qcompare.cpp b/tests/auto/corelib/global/qcompare/tst_qcompare.cpp
new file mode 100644
index 0000000000..61ea0eddc4
--- /dev/null
+++ b/tests/auto/corelib/global/qcompare/tst_qcompare.cpp
@@ -0,0 +1,134 @@
+/****************************************************************************
+**
+** Copyright (C) 2020 Klarälvdalens Datakonsult AB, a KDAB Group company, info@kdab.com, author Giuseppe D'Angelo <giuseppe.dangelo@kdab.com>
+** Contact: http://www.qt.io/licensing/
+**
+** This file is part of the QtCore module of the Qt Toolkit.
+**
+** $QT_BEGIN_LICENSE:LGPL$
+** Commercial License Usage
+** Licensees holding valid commercial Qt licenses may use this file in
+** accordance with the commercial license agreement provided with the
+** Software or, alternatively, in accordance with the terms contained in
+** a written agreement between you and The Qt Company. For licensing terms
+** and conditions see https://www.qt.io/terms-conditions. For further
+** information use the contact form at https://www.qt.io/contact-us.
+**
+** GNU Lesser General Public License Usage
+** Alternatively, this file may be used under the terms of the GNU Lesser
+** General Public License version 3 as published by the Free Software
+** Foundation and appearing in the file LICENSE.LGPL3 included in the
+** packaging of this file. Please review the following information to
+** ensure the GNU Lesser General Public License version 3 requirements
+** will be met: https://www.gnu.org/licenses/lgpl-3.0.html.
+**
+** GNU General Public License Usage
+** Alternatively, this file may be used under the terms of the GNU
+** General Public License version 2.0 or (at your option) the GNU General
+** Public license version 3 or any later version approved by the KDE Free
+** Qt Foundation. The licenses are as published by the Free Software
+** Foundation and appearing in the file LICENSE.GPL2 and LICENSE.GPL3
+** included in the packaging of this file. Please review the following
+** information to ensure the GNU General Public License requirements will
+** be met: https://www.gnu.org/licenses/gpl-2.0.html and
+** https://www.gnu.org/licenses/gpl-3.0.html.
+**
+** $QT_END_LICENSE$
+**
+****************************************************************************/
+
+#include <QtCore/QtCompare>
+#include <QtTest/QTest>
+
+class tst_QCompare: public QObject
+{
+    Q_OBJECT
+private slots:
+    void partialOrdering();
+};
+
+void tst_QCompare::partialOrdering()
+{
+    static_assert(QPartialOrdering::Unordered == QPartialOrdering::Unordered);
+    static_assert(QPartialOrdering::Unordered != QPartialOrdering::Less);
+    static_assert(QPartialOrdering::Unordered != QPartialOrdering::Equivalent);
+    static_assert(QPartialOrdering::Unordered != QPartialOrdering::Greater);
+
+    static_assert(QPartialOrdering::Less != QPartialOrdering::Unordered);
+    static_assert(QPartialOrdering::Less == QPartialOrdering::Less);
+    static_assert(QPartialOrdering::Less != QPartialOrdering::Equivalent);
+    static_assert(QPartialOrdering::Less != QPartialOrdering::Greater);
+
+    static_assert(QPartialOrdering::Equivalent != QPartialOrdering::Unordered);
+    static_assert(QPartialOrdering::Equivalent != QPartialOrdering::Less);
+    static_assert(QPartialOrdering::Equivalent == QPartialOrdering::Equivalent);
+    static_assert(QPartialOrdering::Equivalent != QPartialOrdering::Greater);
+
+    static_assert(QPartialOrdering::Greater != QPartialOrdering::Unordered);
+    static_assert(QPartialOrdering::Greater != QPartialOrdering::Less);
+    static_assert(QPartialOrdering::Greater != QPartialOrdering::Equivalent);
+    static_assert(QPartialOrdering::Greater == QPartialOrdering::Greater);
+
+
+    static_assert(!(QPartialOrdering::Unordered == 0));
+    static_assert(!(QPartialOrdering::Unordered != 0));
+    static_assert(!(QPartialOrdering::Unordered <  0));
+    static_assert(!(QPartialOrdering::Unordered <= 0));
+    static_assert(!(QPartialOrdering::Unordered >  0));
+    static_assert(!(QPartialOrdering::Unordered >= 0));
+
+    static_assert(!(0 == QPartialOrdering::Unordered));
+    static_assert(!(0 != QPartialOrdering::Unordered));
+    static_assert(!(0 <  QPartialOrdering::Unordered));
+    static_assert(!(0 <= QPartialOrdering::Unordered));
+    static_assert(!(0 >  QPartialOrdering::Unordered));
+    static_assert(!(0 >= QPartialOrdering::Unordered));
+
+
+    static_assert(!(QPartialOrdering::Less == 0));
+    static_assert( (QPartialOrdering::Less != 0));
+    static_assert( (QPartialOrdering::Less <  0));
+    static_assert( (QPartialOrdering::Less <= 0));
+    static_assert(!(QPartialOrdering::Less >  0));
+    static_assert(!(QPartialOrdering::Less >= 0));
+
+    static_assert(!(0 == QPartialOrdering::Less));
+    static_assert( (0 != QPartialOrdering::Less));
+    static_assert(!(0 <  QPartialOrdering::Less));
+    static_assert(!(0 <= QPartialOrdering::Less));
+    static_assert( (0 >  QPartialOrdering::Less));
+    static_assert( (0 >= QPartialOrdering::Less));
+
+
+    static_assert( (QPartialOrdering::Equivalent == 0));
+    static_assert(!(QPartialOrdering::Equivalent != 0));
+    static_assert(!(QPartialOrdering::Equivalent <  0));
+    static_assert( (QPartialOrdering::Equivalent <= 0));
+    static_assert(!(QPartialOrdering::Equivalent >  0));
+    static_assert( (QPartialOrdering::Equivalent >= 0));
+
+    static_assert( (0 == QPartialOrdering::Equivalent));
+    static_assert(!(0 != QPartialOrdering::Equivalent));
+    static_assert(!(0 <  QPartialOrdering::Equivalent));
+    static_assert( (0 <= QPartialOrdering::Equivalent));
+    static_assert(!(0 >  QPartialOrdering::Equivalent));
+    static_assert( (0 >= QPartialOrdering::Equivalent));
+
+
+    static_assert(!(QPartialOrdering::Greater == 0));
+    static_assert( (QPartialOrdering::Greater != 0));
+    static_assert(!(QPartialOrdering::Greater <  0));
+    static_assert(!(QPartialOrdering::Greater <= 0));
+    static_assert( (QPartialOrdering::Greater >  0));
+    static_assert( (QPartialOrdering::Greater >= 0));
+
+    static_assert(!(0 == QPartialOrdering::Greater));
+    static_assert( (0 != QPartialOrdering::Greater));
+    static_assert( (0 <  QPartialOrdering::Greater));
+    static_assert( (0 <= QPartialOrdering::Greater));
+    static_assert(!(0 >  QPartialOrdering::Greater));
+    static_assert(!(0 >= QPartialOrdering::Greater));
+}
+
+QTEST_MAIN(tst_QCompare)
+#include "tst_qcompare.moc"
diff --git a/tests/auto/corelib/kernel/qmetatype/tst_qmetatype.cpp b/tests/auto/corelib/kernel/qmetatype/tst_qmetatype.cpp
index 5a4ade5e47..b49f770919 100644
--- a/tests/auto/corelib/kernel/qmetatype/tst_qmetatype.cpp
+++ b/tests/auto/corelib/kernel/qmetatype/tst_qmetatype.cpp
@@ -2576,17 +2576,17 @@ void tst_QMetaType::compareCustomEqualOnlyType()
 
     // check QMetaType::compare works/doesn't crash for equals only comparators
     auto cmp = type.compare(variant50.constData(), variant50.constData());
-    QVERIFY(!cmp);
+    QCOMPARE(cmp, QPartialOrdering::Unordered);
     bool equals = type.equals(variant50.constData(), variant50.constData());
     QVERIFY(equals);
 
     cmp = type.compare(variant100.constData(), variant100x.constData());
-    QVERIFY(!cmp);
+    QCOMPARE(cmp, QPartialOrdering::Unordered);
     equals = type.equals(variant100.constData(), variant100x.constData());
     QVERIFY(equals);
 
     cmp = type.compare(variant50.constData(), variant100.constData());
-    QVERIFY(!cmp);
+    QCOMPARE(cmp, QPartialOrdering::Unordered);
     equals = type.equals(variant50.constData(), variant100.constData());
     QVERIFY(!equals);