I see that the discussion goes to the direction I did not expect.
So maybe the caption should be changed from "partial type definition" to "declaration of methods outside of the class definition". This is my real intent. I think it does not hurt ODR, because ODR requires only one definition and allows several equvalent declarations. If the declarations are not equal, the linker fails just like in case of plain function in C++.
I hope that this feature so simple and attractive, the compilers can do it literally tomorrow.