Q_DECLARE_METATYPE(T); error

[anda_skoa]

Strange thing macro is still defined without the header file..

It is not. Which is the error you got.

Because the compiler saw a function declaration, function name Q_DECLARE_METATYPE, one argument of type Message, but missing return type.
So it complained about the missing return type, said it would assume it and warning you that this is not specified standard behavior.

Cheers,
_