Reading the rather disturbing (albeit refreshingly honest, compared to some other distros) [answer to the FAQ "Can Slackware be recompiled from scratch?"](https://docs.slackware.com/slackware:faq#can_slackware_be_recompiled_from_scratch) got me wondering...
> The “Corresponding Source” for a work in object code form means all the source code needed to generate, install, and (for an executable work) run the object code and to modify the work, including scripts to control those activities.
GPLv2 says something similar:
> The source code for a work means the preferred form of the work for making modifications to it. For an executable work, complete source code means all the source code for all modules it contains, plus any associated interface definition files, plus the scripts used to control compilation and installation of the executable.
In the absence of [reproducible builds](https://en.wikipedia.org/wiki/Reproducible_builds), how is it actually legal for third parties (not the copyright holder) to distribute binaries of GPL-licensed software?
Even if I have the corresponding source code and precisely the same build environment that the distributor built a binary with, if the build process is not reproducible then I cannot actually ***generate*** precisely the same copyrighted ***work in object code form*** which I've received.
The GPL doesn't seem to say anything about how distributing source code and build scripts which can generate a different-but-effectively-equivalent(-but-not-easily-verifiably-so) binary being sufficient to comply with the source code requirement.
So, how is distributing these binaries not copyright infringement?
(Obviously in practice everyone agrees that it is OK to distribute non-reproducible binaries, since most everyone does it, but the answer "the entire free software community just seems to agree that slightly violating the GPL is OK because reproducible builds are too much work" is pretty unsatisfying.)