This compatibility chart provided by GNU appears to state that a GPLv3 program can include a LGPLv2.1-only library. This doesn't make sense to me as the LGPLv2.1 license would have to be compatible with GPLv3.

How is this combination possible?

  • Interesting question! I know LGPLv2.1 can be turned into GPLv3 (via LGPLv2.1 sect. 3), so the combination could certainly be done that way, but when that upgrade strategy is necessary, it is noted elsewhere in the matrix by footnote [7]. The LGPLv2.1-library-into-GPLv3 does not have that footnote, suggesting that it is not necessary to turn the LGPLv2.1 into GPLv3 to distribute this combination. – apsillers Aug 28 '20 at 14:58
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    But what makes you think LGPLv2.1 is not compatible with GPLv3? The FSF compatibility list says that it is. – MadHatter Aug 28 '20 at 15:06

Actually, Section 3 of the LGPLv2.1 seems to be explain this quite clearly:

You may opt to apply the terms of the ordinary GNU General Public License instead of this License to a given copy of the Library. To do this, you must alter all the notices that refer to this License, so that they refer to the ordinary GNU General Public License, version 2, instead of to this License. (If a newer version than version 2 of the ordinary GNU General Public License has appeared, then you can specify that version instead if you wish.) Do not make any other change in these notices.

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