Inspired by a question in the IMSLP "Other" forum, but

[Eric]

much more general...
if a composer today were to write a set of variations on a theme by a (often living) contemporary as 18th century composers used to do every other hour, almost never asking for specific permission in the latter case, would it now be probably a violation of copyright law in most countries and most cases? (I ask literally*, that is, ask a question the better to receive an answer, not as an indirect way of making some sort of point.)

*well, not the "every other hour" part- that's exaggeration... (erm- clearly.)

[ScoreUpdater]

Yes: in the vast majority of cases, the set of variations would be a copyright violation, if the original theme was not in the public domain. This is because the set would be considered a derivative work of the theme, and permission would have to be granted from the original copyright holder to publish the work.

Of course, in the 18th and early 19th centuries, there was no copyright law to restrict composers from using others' themes in their own compositions. Even when copyright acts were passed in the mid-19th century, these were often poorly enforced.