Add type traits to safely determine the existence of comparison operators

Containers often define an operator==() or operator<() which is very useful
for generic code. But those operators can usually not be instantiated if
the template argument doesn't implement the operator.

This sometimes leads to the compiler trying all possible template expansions
and implicit conversions for the type, giving extremely long error
messages. The traits support can be used to safely constrain those
operators.

Being able to safely detect this will also allow us to fold the comparison
support that is currently a large cludge for user types directly into
QMetaType.

Change-Id: Ib84afb5348c3eb0be5161d6ba9d5fe237709c65f
Reviewed-by: Thiago Macieira <thiago.macieira@intel.com>

1 files changed, 97 insertions, 0 deletions

diff --git a/tests/auto/corelib/kernel/qmetatype/tst_qmetatype.cpp b/tests/auto/corelib/kernel/qmetatype/tst_qmetatype.cpp
index 8d4cc05eb9..22d481f5e3 100644
--- a/tests/auto/corelib/kernel/qmetatype/tst_qmetatype.cpp
+++ b/tests/auto/corelib/kernel/qmetatype/tst_qmetatype.cpp
@@ -48,6 +48,103 @@
 
 Q_DECLARE_METATYPE(QMetaType::Type)
 
+namespace CheckTypeTraits
+{
+struct NoOperators
+{
+    int x;
+};
+using Nested = QVector<std::pair<int, QMap<QStringList, QVariant>>>;
+using Nested2 = QVector<std::pair<int, QVector<QPair<QStringList, QVariant>>>>;
+
+// basic types
+static_assert(QTypeTraits::has_operator_equal_v<bool>);
+static_assert(QTypeTraits::has_operator_less_than_v<bool>);
+static_assert(QTypeTraits::has_operator_equal_v<int>);
+static_assert(QTypeTraits::has_operator_less_than_v<int>);
+static_assert(QTypeTraits::has_operator_equal_v<double>);
+static_assert(QTypeTraits::has_operator_less_than_v<double>);
+
+// no comparison operators
+static_assert(!QTypeTraits::has_operator_equal_v<NoOperators>);
+static_assert(!QTypeTraits::has_operator_less_than_v<NoOperators>);
+
+// standard Qt types
+static_assert(QTypeTraits::has_operator_equal_v<QString>);
+static_assert(QTypeTraits::has_operator_less_than_v<QString>);
+static_assert(QTypeTraits::has_operator_equal_v<QVariant>);
+static_assert(!QTypeTraits::has_operator_less_than_v<QVariant>);
+
+// QList
+static_assert(QTypeTraits::has_operator_equal_v<QStringList>);
+static_assert(QTypeTraits::has_operator_less_than_v<QStringList>);
+static_assert(!QTypeTraits::has_operator_equal_v<QList<NoOperators>>);
+static_assert(!QTypeTraits::has_operator_less_than_v<QList<NoOperators>>);
+static_assert(QTypeTraits::has_operator_equal_v<QList<QVariant>>);
+static_assert(!QTypeTraits::has_operator_less_than_v<QList<QVariant>>);
+
+// QPair
+static_assert(QTypeTraits::has_operator_equal_v<QPair<int, QString>>);
+static_assert(QTypeTraits::has_operator_less_than_v<QPair<int, QString>>);
+static_assert(!QTypeTraits::has_operator_equal_v<QPair<int, NoOperators>>);
+static_assert(!QTypeTraits::has_operator_less_than_v<QPair<int, NoOperators>>);
+
+// QMap
+static_assert(QTypeTraits::has_operator_equal_v<QMap<int, QString>>);
+static_assert(!QTypeTraits::has_operator_less_than_v<QMap<int, QString>>);
+static_assert(!QTypeTraits::has_operator_equal_v<QMap<int, NoOperators>>);
+static_assert(!QTypeTraits::has_operator_less_than_v<QMap<int, NoOperators>>);
+
+// QHash
+static_assert(QTypeTraits::has_operator_equal_v<QHash<int, QString>>);
+static_assert(!QTypeTraits::has_operator_less_than_v<QHash<int, QString>>);
+static_assert(!QTypeTraits::has_operator_equal_v<QHash<int, NoOperators>>);
+static_assert(!QTypeTraits::has_operator_less_than_v<QHash<int, NoOperators>>);
+
+// std::vector
+static_assert(QTypeTraits::has_operator_equal_v<std::vector<QString>>);
+static_assert(QTypeTraits::has_operator_less_than_v<std::vector<QString>>);
+static_assert(!QTypeTraits::has_operator_equal_v<std::vector<NoOperators>>);
+static_assert(!QTypeTraits::has_operator_less_than_v<std::vector<NoOperators>>);
+static_assert(QTypeTraits::has_operator_equal_v<std::vector<QVariant>>);
+static_assert(!QTypeTraits::has_operator_less_than_v<std::vector<QVariant>>);
+
+// std::pair
+static_assert(QTypeTraits::has_operator_equal_v<std::pair<int, QString>>);
+static_assert(QTypeTraits::has_operator_less_than_v<std::pair<int, QString>>);
+static_assert(!QTypeTraits::has_operator_equal_v<std::pair<int, NoOperators>>);
+static_assert(!QTypeTraits::has_operator_less_than_v<std::pair<int, NoOperators>>);
+
+// std::tuple
+static_assert(QTypeTraits::has_operator_equal_v<std::tuple<int, QString, double>>);
+static_assert(QTypeTraits::has_operator_less_than_v<std::tuple<int, QString, double>>);
+static_assert(!QTypeTraits::has_operator_equal_v<std::tuple<int, QString, NoOperators>>);
+static_assert(!QTypeTraits::has_operator_less_than_v<std::tuple<int, QString, NoOperators>>);
+
+// std::map
+static_assert(QTypeTraits::has_operator_equal_v<std::map<int, QString>>);
+static_assert(QTypeTraits::has_operator_less_than_v<std::map<int, QString>>);
+static_assert(!QTypeTraits::has_operator_equal_v<std::map<int, NoOperators>>);
+static_assert(!QTypeTraits::has_operator_less_than_v<std::map<int, NoOperators>>);
+
+// nested types
+static_assert(QTypeTraits::has_operator_equal_v<Nested>);
+static_assert(!QTypeTraits::has_operator_less_than_v<Nested>);
+static_assert(QTypeTraits::has_operator_equal_v<std::tuple<int, Nested>>);
+static_assert(!QTypeTraits::has_operator_less_than_v<std::tuple<int, Nested>>);
+static_assert(QTypeTraits::has_operator_equal_v<std::tuple<int, Nested>>);
+static_assert(!QTypeTraits::has_operator_less_than_v<std::tuple<int, Nested>>);
+
+static_assert(QTypeTraits::has_operator_equal_v<Nested2>);
+static_assert(!QTypeTraits::has_operator_less_than_v<Nested2>);
+static_assert(QTypeTraits::has_operator_equal_v<std::tuple<int, Nested2>>);
+static_assert(!QTypeTraits::has_operator_less_than_v<std::tuple<int, Nested2>>);
+static_assert(QTypeTraits::has_operator_equal_v<std::tuple<int, Nested2>>);
+static_assert(!QTypeTraits::has_operator_less_than_v<std::tuple<int, Nested2>>);
+
+}
+
+
 class tst_QMetaType: public QObject
 {
     Q_OBJECT