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>

3 files changed, 291 insertions, 0 deletions

diff --git a/src/corelib/global/global.pri b/src/corelib/global/global.pri
index 4e9a0a0c8b..23b5a284ce 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.
+*/