Q*Ordering: supply is_{lt,gt,n,}eq

std::is_eq etc work fine on our Q*Ordering types when C++20 is
available. But in C++17 mode, they're absent.

Plug the hole by providing them for our own types as hidden friends,
so that users can use unqualified calls to transparently support both
std::*ordering and Q*Ordering types, just like with comparison to
literal zero.

For some reason, we running here into
https://gcc.gnu.org/bugzilla/show_bug.cgi?id=100903, even though this
wasn't seen in other commits of the chain that use comparisons to
literal zero. Suppress the warning. Then at least is_eq etc provide a
safe fall-back for compilers and users affected by this bogus warning.

Fixes: QTBUG-119100
Change-Id: Ie8519c92363401a0c9c8efba6eb0b6e030a8e235
Reviewed-by: Ivan Solovev <ivan.solovev@qt.io>
Reviewed-by: Qt CI Bot <qt_ci_bot@qt-project.org>

3 files changed, 192 insertions, 0 deletions

diff --git a/src/corelib/global/qcompare.h b/src/corelib/global/qcompare.h
index 4bc97fa24e..2c0392097c 100644
--- a/src/corelib/global/qcompare.h
+++ b/src/corelib/global/qcompare.h
@@ -152,6 +152,17 @@ private:
         : m_order(static_cast<QtPrivate::CompareUnderlyingType>(order))
     {}
 
+    QT_WARNING_PUSH
+    // https://gcc.gnu.org/bugzilla/show_bug.cgi?id=100903
+    QT_WARNING_DISABLE_GCC("-Wzero-as-null-pointer-constant")
+    friend constexpr bool is_eq  (QPartialOrdering o) noexcept { return o == 0; }
+    friend constexpr bool is_neq (QPartialOrdering o) noexcept { return o != 0; }
+    friend constexpr bool is_lt  (QPartialOrdering o) noexcept { return o <  0; }
+    friend constexpr bool is_lteq(QPartialOrdering o) noexcept { return o <= 0; }
+    friend constexpr bool is_gt  (QPartialOrdering o) noexcept { return o >  0; }
+    friend constexpr bool is_gteq(QPartialOrdering o) noexcept { return o >= 0; }
+    QT_WARNING_POP
+
     // instead of the exposition only is_ordered member in [cmp.partialord],
     // use a private function
     constexpr bool isOrdered() const noexcept
@@ -309,6 +320,17 @@ private:
         : m_order(static_cast<QtPrivate::CompareUnderlyingType>(order))
     {}
 
+    QT_WARNING_PUSH
+    // https://gcc.gnu.org/bugzilla/show_bug.cgi?id=100903
+    QT_WARNING_DISABLE_GCC("-Wzero-as-null-pointer-constant")
+    friend constexpr bool is_eq  (QWeakOrdering o) noexcept { return o == 0; }
+    friend constexpr bool is_neq (QWeakOrdering o) noexcept { return o != 0; }
+    friend constexpr bool is_lt  (QWeakOrdering o) noexcept { return o <  0; }
+    friend constexpr bool is_lteq(QWeakOrdering o) noexcept { return o <= 0; }
+    friend constexpr bool is_gt  (QWeakOrdering o) noexcept { return o >  0; }
+    friend constexpr bool is_gteq(QWeakOrdering o) noexcept { return o >= 0; }
+    QT_WARNING_POP
+
     QtPrivate::CompareUnderlyingType m_order;
 };
 
@@ -478,6 +500,17 @@ public:
         : m_order(static_cast<QtPrivate::CompareUnderlyingType>(order))
     {}
 
+    QT_WARNING_PUSH
+    // https://gcc.gnu.org/bugzilla/show_bug.cgi?id=100903
+    QT_WARNING_DISABLE_GCC("-Wzero-as-null-pointer-constant")
+    friend constexpr bool is_eq  (QStrongOrdering o) noexcept { return o == 0; }
+    friend constexpr bool is_neq (QStrongOrdering o) noexcept { return o != 0; }
+    friend constexpr bool is_lt  (QStrongOrdering o) noexcept { return o <  0; }
+    friend constexpr bool is_lteq(QStrongOrdering o) noexcept { return o <= 0; }
+    friend constexpr bool is_gt  (QStrongOrdering o) noexcept { return o >  0; }
+    friend constexpr bool is_gteq(QStrongOrdering o) noexcept { return o >= 0; }
+    QT_WARNING_POP
+
     QtPrivate::CompareUnderlyingType m_order;
 };
 
diff --git a/src/corelib/global/qcompare.qdoc b/src/corelib/global/qcompare.qdoc
index ffeb1ec887..ab0f2fb49f 100644
--- a/src/corelib/global/qcompare.qdoc
+++ b/src/corelib/global/qcompare.qdoc
@@ -224,6 +224,30 @@
 */
 
 /*!
+    \fn QStrongOrdering::is_eq  (QStrongOrdering o)
+    \fn QStrongOrdering::is_neq (QStrongOrdering o)
+    \fn QStrongOrdering::is_lt  (QStrongOrdering o)
+    \fn QStrongOrdering::is_lteq(QStrongOrdering o)
+    \fn QStrongOrdering::is_gt  (QStrongOrdering o)
+    \fn QStrongOrdering::is_gteq(QStrongOrdering o)
+
+//! [is_eq_table]
+    Converts \a o into the result of one of the six relational operators:
+    \table
+    \header \li Function    \li Operation
+    \row    \li \c{is_eq}   \li \a o \c{== 0}
+    \row    \li \c{is_neq}  \li \a o \c{!= 0}
+    \row    \li \c{is_lt}   \li \a o \c{< 0}
+    \row    \li \c{is_lteq} \li \a o \c{<= 0}
+    \row    \li \c{is_gt}   \li \a o \c{> 0}
+    \row    \li \c{is_gteq} \li \a o \c{>= 0}
+    \endtable
+//! [is_eq_table]
+
+    These functions are provided for compatibility with \c{std::strong_ordering}.
+*/
+
+/*!
     \variable QStrongOrdering::Less
 
     Represents the result of a comparison where the left operand is less
@@ -367,6 +391,19 @@
 */
 
 /*!
+    \fn QWeakOrdering::is_eq  (QWeakOrdering o)
+    \fn QWeakOrdering::is_neq (QWeakOrdering o)
+    \fn QWeakOrdering::is_lt  (QWeakOrdering o)
+    \fn QWeakOrdering::is_lteq(QWeakOrdering o)
+    \fn QWeakOrdering::is_gt  (QWeakOrdering o)
+    \fn QWeakOrdering::is_gteq(QWeakOrdering o)
+
+    \include qcompare.qdoc is_eq_table
+
+    These functions are provided for compatibility with \c{std::weak_ordering}.
+*/
+
+/*!
     \variable QWeakOrdering::Less
 
     Represents the result of a comparison where the left operand is less than
@@ -500,6 +537,19 @@
 */
 
 /*!
+    \fn QPartialOrdering::is_eq  (QPartialOrdering o)
+    \fn QPartialOrdering::is_neq (QPartialOrdering o)
+    \fn QPartialOrdering::is_lt  (QPartialOrdering o)
+    \fn QPartialOrdering::is_lteq(QPartialOrdering o)
+    \fn QPartialOrdering::is_gt  (QPartialOrdering o)
+    \fn QPartialOrdering::is_gteq(QPartialOrdering o)
+
+    \include qcompare.qdoc is_eq_table
+
+    These functions are provided for compatibility with \c{std::partial_ordering}.
+*/
+
+/*!
     \variable QPartialOrdering::Less
 
     Represents the result of a comparison where the left operand is less than
diff --git a/tests/auto/corelib/global/qcompare/tst_qcompare.cpp b/tests/auto/corelib/global/qcompare/tst_qcompare.cpp
index a8dcc3a201..ac5aa7cb17 100644
--- a/tests/auto/corelib/global/qcompare/tst_qcompare.cpp
+++ b/tests/auto/corelib/global/qcompare/tst_qcompare.cpp
@@ -16,6 +16,7 @@ private slots:
     void weakOrdering();
     void strongOrdering();
     void conversions();
+    void is_eq_overloads();
 };
 
 void tst_QCompare::partialOrdering()
@@ -40,6 +41,12 @@ void tst_QCompare::partialOrdering()
     static_assert(QPartialOrdering::Greater != QPartialOrdering::Equivalent);
     static_assert(QPartialOrdering::Greater == QPartialOrdering::Greater);
 
+    static_assert(!is_eq  (QPartialOrdering::Unordered));
+    static_assert(!is_neq (QPartialOrdering::Unordered));
+    static_assert(!is_lt  (QPartialOrdering::Unordered));
+    static_assert(!is_lteq(QPartialOrdering::Unordered));
+    static_assert(!is_gt  (QPartialOrdering::Unordered));
+    static_assert(!is_gteq(QPartialOrdering::Unordered));
 
     static_assert(!(QPartialOrdering::Unordered == 0));
     static_assert(!(QPartialOrdering::Unordered != 0));
@@ -56,6 +63,13 @@ void tst_QCompare::partialOrdering()
     static_assert(!(0 >= QPartialOrdering::Unordered));
 
 
+    static_assert(!is_eq  (QPartialOrdering::Less));
+    static_assert( is_neq (QPartialOrdering::Less));
+    static_assert( is_lt  (QPartialOrdering::Less));
+    static_assert( is_lteq(QPartialOrdering::Less));
+    static_assert(!is_gt  (QPartialOrdering::Less));
+    static_assert(!is_gteq(QPartialOrdering::Less));
+
     static_assert(!(QPartialOrdering::Less == 0));
     static_assert( (QPartialOrdering::Less != 0));
     static_assert( (QPartialOrdering::Less <  0));
@@ -71,6 +85,13 @@ void tst_QCompare::partialOrdering()
     static_assert( (0 >= QPartialOrdering::Less));
 
 
+    static_assert( is_eq  (QPartialOrdering::Equivalent));
+    static_assert(!is_neq (QPartialOrdering::Equivalent));
+    static_assert(!is_lt  (QPartialOrdering::Equivalent));
+    static_assert( is_lteq(QPartialOrdering::Equivalent));
+    static_assert(!is_gt  (QPartialOrdering::Equivalent));
+    static_assert( is_gteq(QPartialOrdering::Equivalent));
+
     static_assert( (QPartialOrdering::Equivalent == 0));
     static_assert(!(QPartialOrdering::Equivalent != 0));
     static_assert(!(QPartialOrdering::Equivalent <  0));
@@ -86,6 +107,13 @@ void tst_QCompare::partialOrdering()
     static_assert( (0 >= QPartialOrdering::Equivalent));
 
 
+    static_assert(!is_eq  (QPartialOrdering::Greater));
+    static_assert( is_neq (QPartialOrdering::Greater));
+    static_assert(!is_lt  (QPartialOrdering::Greater));
+    static_assert(!is_lteq(QPartialOrdering::Greater));
+    static_assert( is_gt  (QPartialOrdering::Greater));
+    static_assert( is_gteq(QPartialOrdering::Greater));
+
     static_assert(!(QPartialOrdering::Greater == 0));
     static_assert( (QPartialOrdering::Greater != 0));
     static_assert(!(QPartialOrdering::Greater <  0));
@@ -115,6 +143,13 @@ void tst_QCompare::weakOrdering()
     static_assert(QWeakOrdering::Greater != QWeakOrdering::Equivalent);
     static_assert(QWeakOrdering::Greater == QWeakOrdering::Greater);
 
+    static_assert(!is_eq  (QWeakOrdering::Less));
+    static_assert( is_neq (QWeakOrdering::Less));
+    static_assert( is_lt  (QWeakOrdering::Less));
+    static_assert( is_lteq(QWeakOrdering::Less));
+    static_assert(!is_gt  (QWeakOrdering::Less));
+    static_assert(!is_gteq(QWeakOrdering::Less));
+
     static_assert(!(QWeakOrdering::Less == 0));
     static_assert( (QWeakOrdering::Less != 0));
     static_assert( (QWeakOrdering::Less <  0));
@@ -129,6 +164,14 @@ void tst_QCompare::weakOrdering()
     static_assert( (0 >  QWeakOrdering::Less));
     static_assert( (0 >= QWeakOrdering::Less));
 
+
+    static_assert( is_eq  (QWeakOrdering::Equivalent));
+    static_assert(!is_neq (QWeakOrdering::Equivalent));
+    static_assert(!is_lt  (QWeakOrdering::Equivalent));
+    static_assert( is_lteq(QWeakOrdering::Equivalent));
+    static_assert(!is_gt  (QWeakOrdering::Equivalent));
+    static_assert( is_gteq(QWeakOrdering::Equivalent));
+
     static_assert( (QWeakOrdering::Equivalent == 0));
     static_assert(!(QWeakOrdering::Equivalent != 0));
     static_assert(!(QWeakOrdering::Equivalent <  0));
@@ -143,6 +186,14 @@ void tst_QCompare::weakOrdering()
     static_assert(!(0 >  QWeakOrdering::Equivalent));
     static_assert( (0 >= QWeakOrdering::Equivalent));
 
+
+    static_assert(!is_eq  (QWeakOrdering::Greater));
+    static_assert( is_neq (QWeakOrdering::Greater));
+    static_assert(!is_lt  (QWeakOrdering::Greater));
+    static_assert(!is_lteq(QWeakOrdering::Greater));
+    static_assert( is_gt  (QWeakOrdering::Greater));
+    static_assert( is_gteq(QWeakOrdering::Greater));
+
     static_assert(!(QWeakOrdering::Greater == 0));
     static_assert( (QWeakOrdering::Greater != 0));
     static_assert(!(QWeakOrdering::Greater <  0));
@@ -180,6 +231,13 @@ void tst_QCompare::strongOrdering()
     static_assert(QStrongOrdering::Greater != QStrongOrdering::Equivalent);
     static_assert(QStrongOrdering::Greater == QStrongOrdering::Greater);
 
+    static_assert(!is_eq  (QStrongOrdering::Less));
+    static_assert( is_neq (QStrongOrdering::Less));
+    static_assert( is_lt  (QStrongOrdering::Less));
+    static_assert( is_lteq(QStrongOrdering::Less));
+    static_assert(!is_gt  (QStrongOrdering::Less));
+    static_assert(!is_gteq(QStrongOrdering::Less));
+
     static_assert(!(QStrongOrdering::Less == 0));
     static_assert( (QStrongOrdering::Less != 0));
     static_assert( (QStrongOrdering::Less <  0));
@@ -194,6 +252,14 @@ void tst_QCompare::strongOrdering()
     static_assert( (0 >  QStrongOrdering::Less));
     static_assert( (0 >= QStrongOrdering::Less));
 
+
+    static_assert( is_eq  (QStrongOrdering::Equal));
+    static_assert(!is_neq (QStrongOrdering::Equal));
+    static_assert(!is_lt  (QStrongOrdering::Equal));
+    static_assert( is_lteq(QStrongOrdering::Equal));
+    static_assert(!is_gt  (QStrongOrdering::Equal));
+    static_assert( is_gteq(QStrongOrdering::Equal));
+
     static_assert( (QStrongOrdering::Equal == 0));
     static_assert(!(QStrongOrdering::Equal != 0));
     static_assert(!(QStrongOrdering::Equal <  0));
@@ -208,6 +274,14 @@ void tst_QCompare::strongOrdering()
     static_assert(!(0 >  QStrongOrdering::Equal));
     static_assert( (0 >= QStrongOrdering::Equal));
 
+
+    static_assert( is_eq  (QStrongOrdering::Equivalent));
+    static_assert(!is_neq (QStrongOrdering::Equivalent));
+    static_assert(!is_lt  (QStrongOrdering::Equivalent));
+    static_assert( is_lteq(QStrongOrdering::Equivalent));
+    static_assert(!is_gt  (QStrongOrdering::Equivalent));
+    static_assert( is_gteq(QStrongOrdering::Equivalent));
+
     static_assert( (QStrongOrdering::Equivalent == 0));
     static_assert(!(QStrongOrdering::Equivalent != 0));
     static_assert(!(QStrongOrdering::Equivalent <  0));
@@ -222,6 +296,14 @@ void tst_QCompare::strongOrdering()
     static_assert(!(0 >  QStrongOrdering::Equivalent));
     static_assert( (0 >= QStrongOrdering::Equivalent));
 
+
+    static_assert(!is_eq  (QStrongOrdering::Greater));
+    static_assert( is_neq (QStrongOrdering::Greater));
+    static_assert(!is_lt  (QStrongOrdering::Greater));
+    static_assert(!is_lteq(QStrongOrdering::Greater));
+    static_assert( is_gt  (QStrongOrdering::Greater));
+    static_assert( is_gteq(QStrongOrdering::Greater));
+
     static_assert(!(QStrongOrdering::Greater == 0));
     static_assert( (QStrongOrdering::Greater != 0));
     static_assert(!(QStrongOrdering::Greater <  0));
@@ -366,5 +448,32 @@ void tst_QCompare::conversions()
 
 }
 
+void tst_QCompare::is_eq_overloads()
+{
+#ifndef __cpp_lib_three_way_comparison
+    QSKIP("This test requires C++20 three-way-comparison support enabled in the stdlib.");
+#else
+    constexpr auto u = std::partial_ordering::unordered;
+    constexpr auto l = std::weak_ordering::less;
+    constexpr auto g = std::strong_ordering::greater;
+    constexpr auto e = std::weak_ordering::equivalent;
+    constexpr auto s = std::strong_ordering::equal;
+
+    // This is a compile-time check that unqualified name lookup of
+    // std::is_eq-like functions isn't ambiguous, so we can recommend it to our
+    // users for minimizing porting on the way to C++20.
+
+    // The goal is to check each std::ordering and each is_eq function at least
+    // once, not to test all combinations (we're not the stdlib test suite here).
+
+    QVERIFY(is_eq(s));
+    QVERIFY(is_neq(u));
+    QVERIFY(is_lt(l));
+    QVERIFY(is_gt(g));
+    QVERIFY(is_lteq(e));
+    QVERIFY(is_gteq(s));
+#endif // __cpp_lib_three_way_comparison
+}
+
 QTEST_MAIN(tst_QCompare)
 #include "tst_qcompare.moc"