The above copyright notice and this permission notice shall be included in all copies or substantial portions of the Software.
The project is customarily augmented by source code patches. Suppose that someone wished to distribute either their modifications to, or their modified version of, dwm. Without the agreement of Suckless, it is clear to me that any fork of dwm must also be licensed under an MIT/X-compatible license. But since the patches are new material in themselves, it seems to me that the patches themselves could (pathologically) be given an incompatible license, which would forbid anyone from creating or distributing a fork of dwm which included those patches. I make no assumption about how substantial the patches are - they could be a minor change, or they might change the nature of the program completely.
I am curious about several different situations:
Suppose Alice writes a patch of dwm, and explicitly licenses her changes under a restrictive copyright license. Alice could distribute her changes to whoever she likes, but could not distribute a fork of dwm which included those changes. Bob (who is lawfully given Alice's changes) could apply them to dwm, but would not be able to distribute either those changes, or their version of dwm, to anyone else. Is this accurate?
Suppose Alice writes a patch, and does not explicitly license it at all. If we assume nothing at all about Alice's work, then the copyright is hers and hers alone, and we are in the same situation as 1. Suppose that it is "known" that Alice's work is intended to be a patch of dwm. Does the default situation change in this case, or does it require an explicit agreement of Alice?
Suppose Alice writes a patch, and explicitly licenses it under a license like the GPL, which also includes a clause that requires modified versions to be under the same license. Are forked versions forbidden in practice, just like in situation 1?
These restrictions sound like a major obstacle to patching open source programs, and require agreement of all contributors to submit to the parent license. Are such agreements common in projects which accept contributions?
: Alice could of course distribute a fork of dwm provided that it included a copy of the MIT/X License. I believe this would in practice cause the changes themselves to be subject to that same license, effectively nullifying her simultaneous restrictive license.
: By "known", I mean any precise criterion which would indicate that a piece of software is intended for use as a modification to an existing program, and not as a new project in and of itself. For example, if the official host page of the patch said something to the effect of "This file consists of a patch to version 6.1 of dwm". Something which is a direct mention that the work is a modification, but not which makes any specific statements about the copyright of the work.
: I am under the impression that code cannot simultaneously be licensed under MIT/X and GPL.