Make QMetaTypeInterface constexpr on Windows

This was so far problematic as it gave various link errors. The solution
to that seems to be to make the default constructor of QPairVariantInterfaceImpl
constexpr to get around one set of problems.

The other problem to solve where undefined references to metaobjects. The
reason for that is apparently that QMetaTypeInterface contains a direct
pointer to the meta object, something the linker doesn't like. Adding a
level of indirection by using a function that returns the pointer seems
to solve that problem.

Fixes: QTBUG-88468
Change-Id: I5612ae807ea3b7e49bc40349d8d1fca1be9bd7ee
Reviewed-by: Fabian Kosmale <fabian.kosmale@qt.io>

2 files changed, 63 insertions, 56 deletions

diff --git a/src/corelib/kernel/qmetatype.h b/src/corelib/kernel/qmetatype.h
index 86cd15def8..8ef1bb8faa 100644
--- a/src/corelib/kernel/qmetatype.h
+++ b/src/corelib/kernel/qmetatype.h
@@ -698,7 +698,7 @@ public:
     {
     }
 
-    QPairVariantInterfaceImpl()
+    constexpr QPairVariantInterfaceImpl()
       : _pair(nullptr)
       , _getFirst(nullptr)
       , _getSecond(nullptr)
@@ -818,36 +818,44 @@ namespace QtPrivate
     template<typename T, typename Enable = void>
     struct MetaObjectForType
     {
-        static constexpr inline const QMetaObject *value() { return nullptr; }
+        static constexpr const QMetaObject *value() { return nullptr; }
+        using MetaObjectFn = const QMetaObject *(*)(const QMetaTypeInterface *);
+        static constexpr MetaObjectFn metaObjectFunction = nullptr;
     };
 #ifndef QT_NO_QOBJECT
     template<>
     struct MetaObjectForType<void>
     {
-        static constexpr inline const QMetaObject *value() { return nullptr; }
+        static constexpr const QMetaObject *value() { return nullptr; }
+        using MetaObjectFn = const QMetaObject *(*)(const QMetaTypeInterface *);
+        static constexpr MetaObjectFn metaObjectFunction = nullptr;
     };
     template<typename T>
     struct MetaObjectForType<T*, typename std::enable_if<IsPointerToTypeDerivedFromQObject<T*>::Value>::type>
     {
-        static constexpr inline const QMetaObject *value() { return &T::staticMetaObject; }
+        static constexpr const QMetaObject *value() { return &T::staticMetaObject; }
+        static constexpr const QMetaObject *metaObjectFunction(const QMetaTypeInterface *) { return &T::staticMetaObject; }
     };
     template<typename T>
     struct MetaObjectForType<T, typename std::enable_if<IsGadgetHelper<T>::IsGadgetOrDerivedFrom>::type>
     {
-        static constexpr inline const QMetaObject *value() { return &T::staticMetaObject; }
+        static constexpr const QMetaObject *value() { return &T::staticMetaObject; }
+        static constexpr const QMetaObject *metaObjectFunction(const QMetaTypeInterface *) { return &T::staticMetaObject; }
     };
     template<typename T>
     struct MetaObjectForType<T, typename std::enable_if<IsPointerToGadgetHelper<T>::IsGadgetOrDerivedFrom>::type>
     {
-        static constexpr inline const QMetaObject *value()
+        static constexpr const QMetaObject *value()
         {
             return &IsPointerToGadgetHelper<T>::BaseType::staticMetaObject;
         }
+        static constexpr const QMetaObject *metaObjectFunction(const QMetaTypeInterface *) { return value(); }
     };
     template<typename T>
     struct MetaObjectForType<T, typename std::enable_if<IsQEnumHelper<T>::Value>::type >
     {
-        static constexpr inline const QMetaObject *value() { return qt_getEnumMetaObject(T()); }
+        static constexpr const QMetaObject *value() { return qt_getEnumMetaObject(T()); }
+        static constexpr const QMetaObject *metaObjectFunction(const QMetaTypeInterface *) { return value(); }
     };
 #endif
 
@@ -1590,7 +1598,10 @@ public:
     uint size;
     uint flags;
     mutable QBasicAtomicInt typeId;
-    const QMetaObject *metaObject;
+
+    using MetaObjectFn = const QMetaObject *(*)(const QMetaTypeInterface *);
+    const MetaObjectFn metaObjectFn;
+
     const char *name;
 
     using DefaultCtrFn = void (*)(const QMetaTypeInterface *, void *);
@@ -2224,42 +2235,27 @@ public:
 template<typename T>
 struct QMetaTypeInterfaceWrapper
 {
-    static inline constexpr QMetaTypeInterface create()
-    {
-        return {
-            /*.revision=*/ 0,
-            /*.alignment=*/ alignof(T),
-            /*.size=*/ sizeof(T),
-            /*.flags=*/ QMetaTypeTypeFlags<T>::Flags,
-            /*.typeId=*/ BuiltinMetaType<T>::value,
-            /*.metaObject=*/ MetaObjectForType<T>::value(),
-            /*.name=*/ QMetaTypeForType<T>::getName(),
-            /*.defaultCtr=*/ QMetaTypeForType<T>::getDefaultCtr(),
-            /*.copyCtr=*/ QMetaTypeForType<T>::getCopyCtr(),
-            /*.moveCtr=*/ QMetaTypeForType<T>::getMoveCtr(),
-            /*.dtor=*/ QMetaTypeForType<T>::getDtor(),
-            /*.equals=*/ QEqualityOperatorForType<T>::equals,
-            /*.lessThan=*/ QLessThanOperatorForType<T>::lessThan,
-            /*.debugStream=*/ QDebugStreamOperatorForType<T>::debugStream,
-            /*.dataStreamOut=*/ QDataStreamOperatorForType<T>::dataStreamOut,
-            /*.dataStreamIn=*/ QDataStreamOperatorForType<T>::dataStreamIn,
-            /*.legacyRegisterOp=*/ QMetaTypeForType<T>::getLegacyRegister()
-        };
-    }
-
-#ifdef Q_OS_WIN
-    // MSVC produces link errors when the metaType is constexpr
-    static const QMetaTypeInterface metaType;
-#else
-    static constexpr const QMetaTypeInterface metaType = create();
-#endif
+    static inline constexpr const QMetaTypeInterface metaType = {
+        /*.revision=*/ 0,
+        /*.alignment=*/ alignof(T),
+        /*.size=*/ sizeof(T),
+        /*.flags=*/ QMetaTypeTypeFlags<T>::Flags,
+        /*.typeId=*/ BuiltinMetaType<T>::value,
+        /*.metaObjectFn=*/ MetaObjectForType<T>::metaObjectFunction,
+        /*.name=*/ QMetaTypeForType<T>::getName(),
+        /*.defaultCtr=*/ QMetaTypeForType<T>::getDefaultCtr(),
+        /*.copyCtr=*/ QMetaTypeForType<T>::getCopyCtr(),
+        /*.moveCtr=*/ QMetaTypeForType<T>::getMoveCtr(),
+        /*.dtor=*/ QMetaTypeForType<T>::getDtor(),
+        /*.equals=*/ QEqualityOperatorForType<T>::equals,
+        /*.lessThan=*/ QLessThanOperatorForType<T>::lessThan,
+        /*.debugStream=*/ QDebugStreamOperatorForType<T>::debugStream,
+        /*.dataStreamOut=*/ QDataStreamOperatorForType<T>::dataStreamOut,
+        /*.dataStreamIn=*/ QDataStreamOperatorForType<T>::dataStreamIn,
+        /*.legacyRegisterOp=*/ QMetaTypeForType<T>::getLegacyRegister()
+    };
 };
 
-#ifdef Q_OS_WIN
-template<typename T>
-const QMetaTypeInterface QMetaTypeInterfaceWrapper<T>::metaType
-    = QMetaTypeInterfaceWrapper<T>::create();
-#endif
 
 template<>
 class QMetaTypeInterfaceWrapper<void>
@@ -2272,7 +2268,7 @@ public:
         /*.size=*/ 0,
         /*.flags=*/ 0,
         /*.typeId=*/ BuiltinMetaType<void>::value,
-        /*.metaObject=*/ nullptr,
+        /*.metaObjectFn=*/ nullptr,
         /*.name=*/ "void",
         /*.defaultCtr=*/ nullptr,
         /*.copyCtr=*/ nullptr,
@@ -2395,7 +2391,7 @@ constexpr QMetaType::TypeFlags QMetaType::flags() const
 
 constexpr const QMetaObject *QMetaType::metaObject() const
 {
-    return d_ptr ? d_ptr->metaObject : nullptr;
+    return d_ptr && d_ptr->metaObjectFn ? d_ptr->metaObjectFn(d_ptr) : nullptr;
 }
 
 template<typename... T>
diff --git a/tests/auto/corelib/kernel/qmetatype/tst_qmetatype.cpp b/tests/auto/corelib/kernel/qmetatype/tst_qmetatype.cpp
index f3d95285dc..503e9b0e33 100644
--- a/tests/auto/corelib/kernel/qmetatype/tst_qmetatype.cpp
+++ b/tests/auto/corelib/kernel/qmetatype/tst_qmetatype.cpp
@@ -413,19 +413,30 @@ void tst_QMetaType::registerGadget(const char *name, const QList<GadgetPropertyT
     meta->d.static_metacall = &GadgetsStaticMetacallFunction;
     meta->d.superdata = nullptr;
     const auto flags = QMetaType::IsGadget | QMetaType::NeedsConstruction | QMetaType::NeedsDestruction;
-    using TypeInfo = QtPrivate::QMetaTypeInterface;
+    struct TypeInfo : public QtPrivate::QMetaTypeInterface
+    {
+        QMetaObject *mo;
+    };
+
     auto typeInfo = new TypeInfo {
-        0, alignof(GenericGadgetType), sizeof(GenericGadgetType), uint(flags), 0, meta, name,
-        [](const TypeInfo *self, void *where) { GadgetTypedConstructor(self->typeId, where, nullptr); },
-        [](const TypeInfo *self, void *where, const void *copy) { GadgetTypedConstructor(self->typeId, where, copy); },
-        [](const TypeInfo *self, void *where, void *copy) { GadgetTypedConstructor(self->typeId, where, copy); },
-        [](const TypeInfo *self, void *ptr) { GadgetTypedDestructor(self->typeId, ptr); },
-        nullptr,
-        nullptr,
-        nullptr,
-        GadgetSaveOperator,
-        GadgetLoadOperator,
-        nullptr
+        {
+            0, alignof(GenericGadgetType), sizeof(GenericGadgetType), uint(flags), 0,
+            [](const QtPrivate::QMetaTypeInterface *self) -> const QMetaObject * {
+                return reinterpret_cast<const TypeInfo *>(self)->mo;
+            },
+            name,
+            [](const QtPrivate::QMetaTypeInterface *self, void *where) { GadgetTypedConstructor(self->typeId, where, nullptr); },
+            [](const QtPrivate::QMetaTypeInterface *self, void *where, const void *copy) { GadgetTypedConstructor(self->typeId, where, copy); },
+            [](const QtPrivate::QMetaTypeInterface *self, void *where, void *copy) { GadgetTypedConstructor(self->typeId, where, copy); },
+            [](const QtPrivate::QMetaTypeInterface *self, void *ptr) { GadgetTypedDestructor(self->typeId, ptr); },
+            nullptr,
+            nullptr,
+            nullptr,
+            GadgetSaveOperator,
+            GadgetLoadOperator,
+            nullptr
+        },
+        meta
     };
     QMetaType gadgetMetaType(typeInfo);
     dynamicGadgetProperties->m_metatype = gadgetMetaType;