Why does -fsanitize=undefined cause "undefined reference to typeinfo"?

The answer to the actual question as asked is that -fsanitize=undefined is actually a collection of sanitizers including the vptr sanitizer.

https://clang.llvm.org/docs/UndefinedBehaviorSanitizer.html

The vptr sanitizer is clearly marked as requiring RTTI, which other answers have described why it's not available.

To run all the tests except vptr, you can say

-fsanitize=undefined -fno-sanitize=vptr

I think the -fsanitize=undefined is a red herring.

You are only exporting the member functions of that class. In order to also export its metadata (like its typeinfo and potential v-table pointer) you need to export the class.

Try this

class EXPORT MyObject : public QObject
{
public:
    MyObject (QObject * parent = nullptr);
    ~MyObject ();

    void myMethod ();
};

Then you should not need to mark up the individual member functions.