LGPL 2.1 + GPL 3 = problems?

[Philippe Verdy]

Arnoud Engelfriet wrote:
> Jesse Hannah wrote:
> > But I could license the LGPL v2.1 parts under the LGPL v3:
> >
> >     This library is free software; you can redistribute it and/or
> >     modify it under the terms of the GNU Lesser General Public
> >     License as published by the Free Software Foundation; either
> >     version 2.1 of the License, ***or (at your option) any later
> > version.***
> 
> If the library says that, then yes. But not all LGPL libraries carry
> that particular message.

True.

> > The way I'm reading that, I'm free to redistribute the library under
> > the LGPL v3 if I want to, meaning (presumably) that I can use it with
> > a GPLv3 program. Does that sound right?
> 
> Yes, because the LGPLv3 allows conversion of the license to GPLv3.
> With that conversion you can comply with GPLv3 for the entire work.

Wrong ! The LGPGL does NOT authorize the conversion of the licences covering
other necessary components. The terms that allows such promotion are those
found in the licences of those components.

What the LGPL says is that your modified library as a whole, licenced under
LGPL v3 can be also licenced under GPLv3 only (excluding any later version
of the GPL), and in that case, the promotion of LGPL to GPL makes an
explicit "additional restriction" to the default terms of the GPLv3 which
normally allows by default "version 3.0 of the GPL or any later version".

This explicit restriction found in the LGPL v2.1 or in LGPL v3 only applies
to the library as a whole, not to the individual parts of it that may be
covered individually by other licences (such as a prior version of the LGPL
or public domain).