Support Q_GADGET QMetaObject super class hierarchies across templates

This patch fixes the QMetaObject::superClass hierarchy for Q_GADGETs
that inherit from a template which in turn inherits another Q_GADGET.
One common scenario where this is applied is for the CRTP. Without this
patch, moc would stop at the template and then sets the superClass
QMetaObject to a nullptr. For QObjects this works, since there moc knows
that every child must by definition inherit QObject. In order to support
this for Q_GADGETs too, we defer the judgment about the availability
of a staticMetaObject in the base class to compile time through the
existing QtPrivate::MetaObjectForType<Base>::value() helper.

[ChangeLog][QtCore][moc] Moc now correctly sets a non-null
QMetaObject::superClass for Q_GADGETs that inherit from a template which
inherits another Q_GADGET.

Change-Id: I103b5efd74ed24172dffce477ca2ed6d0f374d44
Reviewed-by: Olivier Goffart (Woboq GmbH) <ogoffart@woboq.com>

4 files changed, 17 insertions, 2 deletions

diff --git a/src/tools/moc/generator.cpp b/src/tools/moc/generator.cpp
index 553e16d472..9029bf9008 100644
--- a/src/tools/moc/generator.cpp
+++ b/src/tools/moc/generator.cpp
@@ -541,8 +541,10 @@ void Generator::generateCode()
 
     if (isQObject)
         fprintf(out, "    nullptr,\n");
-    else if (cdef->superclassList.size() && (!cdef->hasQGadget || knownGadgets.contains(purestSuperClass)))
+    else if (cdef->superclassList.size() && !cdef->hasQGadget) // for qobject, we know the super class must have a static metaobject
         fprintf(out, "    QMetaObject::SuperData::link<%s::staticMetaObject>(),\n", purestSuperClass.constData());
+    else if (cdef->superclassList.size()) // for gadgets we need to query at compile time for it
+        fprintf(out, "    QtPrivate::MetaObjectForType<%s>::value(),\n", purestSuperClass.constData());
     else
         fprintf(out, "    nullptr,\n");
     fprintf(out, "    qt_meta_stringdata_%s.data,\n"
diff --git a/tests/auto/tools/moc/allmocs_baseline_in.json b/tests/auto/tools/moc/allmocs_baseline_in.json
index bde5a1c52b..8cb397190c 100644
--- a/tests/auto/tools/moc/allmocs_baseline_in.json
+++ b/tests/auto/tools/moc/allmocs_baseline_in.json
@@ -2061,6 +2061,17 @@
                         "name": "Derived"
                     }
                 ]
+            },
+            {
+                "className": "CRTPDerivedGadget",
+                "gadget": true,
+                "qualifiedClassName": "GrandParentGadget::CRTPDerivedGadget",
+                "superClasses": [
+                    {
+                        "access": "public",
+                        "name": "CRTP<CRTPDerivedGadget>"
+                    }
+                ]
             }
         ],
         "inputFile": "grand-parent-gadget-class.h",
diff --git a/tests/auto/tools/moc/grand-parent-gadget-class.h b/tests/auto/tools/moc/grand-parent-gadget-class.h
index 7ffd636ca1..c7720a1ab9 100644
--- a/tests/auto/tools/moc/grand-parent-gadget-class.h
+++ b/tests/auto/tools/moc/grand-parent-gadget-class.h
@@ -36,7 +36,8 @@ namespace GrandParentGadget {
 struct BaseGadget { Q_GADGET };
 struct Derived : BaseGadget {};
 struct DerivedGadget : Derived { Q_GADGET };
-
+template<typename T> struct CRTP : BaseGadget {};
+struct CRTPDerivedGadget : CRTP<CRTPDerivedGadget> { Q_GADGET };
 }
 
 #endif // GRANDPARENTGADGETCLASS_H
diff --git a/tests/auto/tools/moc/tst_moc.cpp b/tests/auto/tools/moc/tst_moc.cpp
index 9fc00288fe..6b202f79a6 100644
--- a/tests/auto/tools/moc/tst_moc.cpp
+++ b/tests/auto/tools/moc/tst_moc.cpp
@@ -3836,6 +3836,7 @@ void tst_Moc::gadgetHierarchy()
 {
     QCOMPARE(NonGadgetParent::Derived::staticMetaObject.superClass(), static_cast<const QMetaObject*>(nullptr));
     QCOMPARE(GrandParentGadget::DerivedGadget::staticMetaObject.superClass(), &GrandParentGadget::BaseGadget::staticMetaObject);
+    QCOMPARE(GrandParentGadget::CRTPDerivedGadget::staticMetaObject.superClass(), &GrandParentGadget::BaseGadget::staticMetaObject);
 }
 
 void tst_Moc::optionsFileError_data()