I have a class B that I want to declarativly expose to QML. B is a subclass of A.

#include <a.h>

class B : public A {
  Q_OBJECT
  QML_ELEMENT
};

A is defined in a separate library that is installed into system dirs

class A : public QObject {
  Q_OBJECT
}

When building my project I get
Warning: b.h:: A is used but cannot be found.

Looking into the generated qmltypes file B has type information, but is missing all the inherited properties/signals/invokables from A.

I suspect the answer here is to call qt_extract_metatypes() on the library where A is defined. But that will add the metatypes file to the build directory of that library, which doesn't make it available when building the project that contains B. For that it would need to be installed to the system as part of the library's installation process, but as far as I can tell that doesn't happen and there seems to be no API to do that.