Add constexpr template specializations for built in metatypes.

This will make it possible (in Qt 6) to remove the enums listing
metatype ids. As it is constexpr, it can be used in switch statements
just like enums, as enum values, and as template specialization values.

Change-Id: I51293674c403714e34cb8a8b8953522fc97a740a
Reviewed-by: Olivier Goffart <ogoffart@woboq.com>

1 files changed, 41 insertions, 0 deletions

diff --git a/tests/auto/corelib/kernel/qmetatype/tst_qmetatype.cpp b/tests/auto/corelib/kernel/qmetatype/tst_qmetatype.cpp
index b61625714b..c73c26a9ef 100644
--- a/tests/auto/corelib/kernel/qmetatype/tst_qmetatype.cpp
+++ b/tests/auto/corelib/kernel/qmetatype/tst_qmetatype.cpp
@@ -107,6 +107,7 @@ private slots:
     void saveAndLoadBuiltin();
     void saveAndLoadCustom();
     void metaObject();
+    void constexprMetaTypeIds();
 };
 
 struct Foo { int i; };
@@ -1632,6 +1633,46 @@ void tst_QMetaType::metaObject()
     QCOMPARE(QMetaType(QMetaType::Int).metaObject(), static_cast<const QMetaObject *>(0));
 }
 
+#define METATYPE_ID_FUNCTION(Type, MetaTypeId, Name) \
+  case ::qMetaTypeId< Name >(): metaType = MetaTypeIdStruct<MetaTypeId>::Value;
+
+template<int>
+struct MetaTypeIdStruct
+{
+};
+
+#define METATYPE_ID_STRUCT(Type, MetaTypeId, Name) \
+template<> \
+struct MetaTypeIdStruct< ::qMetaTypeId< Name >()> \
+{ \
+    enum { Value = ::qMetaTypeId< Name >() }; \
+};
+
+#if defined(Q_COMPILER_CONSTEXPR)
+QT_FOR_EACH_STATIC_TYPE(METATYPE_ID_STRUCT)
+
+template<int i = ::qMetaTypeId<int>()>
+struct MetaTypeIdStructDefaultTemplateValue
+{
+  enum { Value };
+};
+#endif
+
+void tst_QMetaType::constexprMetaTypeIds()
+{
+    int id = 0;
+    int metaType;
+
+    switch(id) {
+#if defined(Q_COMPILER_CONSTEXPR)
+      QT_FOR_EACH_STATIC_TYPE(METATYPE_ID_FUNCTION)
+      metaType = MetaTypeIdStructDefaultTemplateValue<>::Value;
+#endif
+    default:;
+    }
+    Q_UNUSED(metaType);
+}
+
 // Compile-time test, it should be possible to register function pointer types
 class Undefined;