> You wouldn't have to. Without copyright, it seems to me that everything becomes copyleft by default.
No, without copyright, you cannot force the other party to give you access to the original source code.
BTW: There does not seem to exist an open source license that tries to emulate "no copyright"/"no IP". I.e. the license allows reverse engineering of all binaries derived from it virally, create modifications etc. and perhaps allows to use patents; but the license does not give you the right to demand access to the original source code.
No, without copyright, you cannot force the other party to give you access to the original source code.
BTW: There does not seem to exist an open source license that tries to emulate "no copyright"/"no IP". I.e. the license allows reverse engineering of all binaries derived from it virally, create modifications etc. and perhaps allows to use patents; but the license does not give you the right to demand access to the original source code.