QMetaTypeModuleHelper: fix -Wweak-vtable

The only implemented virtual function, convert(), is inline, so the
vtable and type_info for this class were duplicated in QtGui,
QtWidgets, and any other library that may use this.

Fix by exporting the class and de-inlining convert(). The vtable is
now pinned to qmetatype.cpp.

To prevent MSVC from exporting the trivial static helper function and
possibly rendering its constexpr non-functional, make it a template.

We have two macros for this purpose, with different semantics: The
Q_WEAK_OVERLOAD macro is related to overload set management, not to
keeping function out of the ABI, even though it does that, too. And we
have QT_POST_CXX17_API_IN_EXPORTED_CLASS for ABI control, but this
function is not using post-C++17 features, so since both macros need a
comment, and both don't fit 100%, I used the shorter one.

Task-number: QTBUG-45582
Pick-to: 6.7
Change-Id: I2ce4a7110e09def1a595d717c073df844213611c
Reviewed-by: Thiago Macieira <thiago.macieira@intel.com>

2 files changed, 9 insertions, 2 deletions

diff --git a/src/corelib/kernel/qmetatype.cpp b/src/corelib/kernel/qmetatype.cpp
index a1310992f4..ebe79e0232 100644
--- a/src/corelib/kernel/qmetatype.cpp
+++ b/src/corelib/kernel/qmetatype.cpp
@@ -948,6 +948,12 @@ void QMetaType::unregisterMetaType(QMetaType type)
     than the QMetaType \a b, otherwise returns \c false.
 */
 
+/*! \internal */
+bool QMetaTypeModuleHelper::convert(const void *, int, void *, int) const
+{
+    return false;
+}
+
 #define QT_ADD_STATIC_METATYPE(MetaTypeName, MetaTypeId, RealName) \
     { #RealName, sizeof(#RealName) - 1, MetaTypeId },
 
diff --git a/src/corelib/kernel/qmetatype_p.h b/src/corelib/kernel/qmetatype_p.h
index 71a225d5e6..7e0457771f 100644
--- a/src/corelib/kernel/qmetatype_p.h
+++ b/src/corelib/kernel/qmetatype_p.h
@@ -38,20 +38,21 @@ QT_BEGIN_NAMESPACE
             assign_and_return \
         }
 
-class QMetaTypeModuleHelper
+class Q_CORE_EXPORT QMetaTypeModuleHelper
 {
     Q_DISABLE_COPY_MOVE(QMetaTypeModuleHelper)
 protected:
     QMetaTypeModuleHelper() = default;
     ~QMetaTypeModuleHelper() = default;
 public:
+    Q_WEAK_OVERLOAD // prevent it from entering the ABI and rendering constexpr useless
     static constexpr auto makePair(int from, int to) -> quint64
     {
         return (quint64(from) << 32) + quint64(to);
     }
 
     virtual const QtPrivate::QMetaTypeInterface *interfaceForType(int) const = 0;
-    virtual bool convert(const void *, int, void *, int) const { return false; }
+    virtual bool convert(const void *, int, void *, int) const;
 };
 
 extern Q_CORE_EXPORT const QMetaTypeModuleHelper *qMetaTypeGuiHelper;