Looking Forward to It

> This seems to be different in the EU: I don't think the US has an equivalent to recitals 10 and 15 of Directive EC 2009/24, which, according to the EUPL drafters means strong-copyleft licenses can't exist in the EU.

Recitals aren't operational, they just merely serve to explain the articles. They're probably relying on Article 1(2).

> Article 1(2): [...] Ideas and principles which underlie any element of a computer program, including those which underlie its *interfaces*, are not protected by copyright under this Directive.

If interfaces are not copyrightable, the argument goes, then anything merely using the interface can't be copying any protected expression.

The main issue is that a lot of (online) discussion uses terms like "derivative work" which is not a thing in EU copyright law. The question is: has protected expression from one work has been incorporated into another? That's fundamentally a case-by-case issue. But it seems likely that by using libreadline your program is copying some protected expression from the library. But IANAL.