In a recent article on FSF.org

** The main license in actual use that isn't reasonably behaved is the license of TeX: if two programs are licensed just the way TeX is, there is no authorized way to distribute a merged version of them.

The TeX license permits distribution of a modified version only in the form of the original version plus a differences file. If A and B are separately released that way, then merged, distributing the merged program as A plus a change file violates the license of B. Distributing this as B plus a change file violates the license of A. Distributing this in any other way violates both licenses.

It is no coincidence that TeX was released in 1982: our community has learned, since then, to write reasonably behaved licenses.

Do modern TeX distribution suffer from this issue, or is that just ancient history these days?

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    That is precisely what Knuth intended, to prevent the spread of not-quite-compatible versions of programs that were all called TeX. Of course it doesn't fit with Stallman's entrenched views about how the software industry should operate. – alephzero Nov 23 '18 at 10:29
  • @alephzero Sure: if you look at newer software, though, people use trademarks for the naming, and leave licensing to deal with things it is good at! – Joseph Wright Nov 23 '18 at 10:34
  • For the LaTeX license there is latex-project.org/publications/… and hundreds of mails at lists.debian.org/debian-legal/2002/07 – moewe Nov 23 '18 at 10:34
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    You are given a link that helps you understand the current situation and yet you don't want to read. To sum it up in short: it is complicated. – Johannes_B Nov 23 '18 at 14:38
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    @einpoklum but basically you are asking for legal advice on current licences, i think this is off topic. the licence on tex is as it was so the situation is unchanged but whether there is an issue or anyone suffers is a matter of opinion so also off topic. – David Carlisle Nov 23 '18 at 19:50