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I am wondering whether Creative Commons ShareAlike is a weak or strong copyleft license. That is, does it allow linking with code with under a different license?

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    Is this for your code? Do not use Creative Commons for code. Use CC for content and a source code license for source code. Please clarify if this is for new code or for third party code. – 13042 Sep 13 at 9:12
  • @john: I was just curious while reading about licenses and was surprised I couldn't find any info on this. No particular use case. – nijoakim Sep 15 at 12:29
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    When CC licenses were developed, the problem was that there was a solid system of Free and Open Source software licenses (so no need for complicate further an ecosystem which was working well enough) but there was no applicable, comparable, licensing system for content. That's the reason CC licenses were born, that's the reason CC licenses don't write about software source code, that's the reason CC licenses don't suit software source code well, ultimately being the reason you wouldn't want to use them for software source code! – 13042 Sep 15 at 12:34
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    There's no substantial reason you shouldn't use CC BY-SA for code (its "problems" also apply to licenses like the MIT). – curiousdannii Sep 16 at 4:12
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TL;DR

About one of your questions:

[...] does it allow linking with code with under a different license?

Yes.

It is restriction 4.a applied to grant 3.a supporting on the fitting that linking software libraries has in definition 1.a - read the TS;DR section for the full explanation.

Reference: https://creativecommons.org/licenses/by-sa/2.0/legalcode

About your other question:

I am wondering whether Creative Commons ShareAlike is a weak or strong copyleft license. [...]

Accepting the definitions at Wikipedia (emphasis mine):

The strength of the copyleft governing a work is an expression of the extent that the copyleft provisions can be efficiently imposed on all kinds of derived works. "Weak copyleft" refers to licenses where not all derived works inherit the copyleft license; whether a derived work inherits or not often depends on the manner in which it was derived.

Considering restriction 4.a of CC BY-SA 2.0 (emphasis mine):

You may distribute [...] the Work only under the terms of this License, and You must include a copy of, or the Uniform Resource Identifier for, this License with every copy [...] of the Work You distribute [...]. You may not offer or impose any terms on the Work that alter or restrict the terms of this License or the recipients' exercise of the rights granted hereunder. You may not sublicense the Work. You must keep intact all notices that refer to this License and to the disclaimer of warranties. You may not distribute [...] the Work with any technological measures that control access or use of the Work in a manner inconsistent with the terms of this License Agreement. The above applies to the Work as incorporated in a Collective Work, but this does not require the Collective Work apart from the Work itself to be made subject to the terms of this License. [...]

It explicitly mandates the licensed Work to be weak copyleft.

TS;DR

Definition 1.a

  1. Definitions

    a. "Collective Work" means a work, such as a periodical issue, anthology or encyclopedia, in which the Work in its entirety in unmodified form, along with a number of other contributions, constituting separate and independent works in themselves, are assembled into a collective whole. A work that constitutes a Collective Work will not be considered a Derivative Work (as defined below) for the purposes of this License.

People don't normally develop software on its own, siloed, NIH. Most software will depend on libraries, which, if not tailored, customised to the user project's needs, will be unmodified, hence fitting the category of Collective Work as per CC BY-SA definitions.

When linking to a third party library, one is not creating a Derivative Work, but a Collective Work. However, if any modification is done on the linked library, it becomes Derivative Work.

Note that using actual software licenses, such as e.g. the GNU recommendation, "linking, linked libraries" will be expressions used in the license, rendering needless an interpretation as in the case of CC BY-SA. This is a reason supporting the typical advice "don't use CC licenses for source code". For a graver reason, consider that restriction 4.a of CC BY-SA 2.0 forbids sub licensing all or part of the Work, the code, effectively locking as CC it in case anyone interested would want to fork a GPL alternative from your Work. Now how does that compare with clause 2 of ALv2 explicitly allowing sub licensing!

Grant 3.a

  1. License Grant. Subject to the terms and conditions of this License, Licensor hereby grants You a worldwide, royalty-free, non-exclusive, perpetual (for the duration of the applicable copyright) license to exercise the rights in the Work as stated below:

    a. to reproduce the Work, to incorporate the Work into one or more Collective Works, and to reproduce the Work as incorporated in the Collective Works;

Licensor of some software licensed CC BY-SA 2.0 grants the right to incorporate the Work, e.g. a library; into Collective Works, e.g. a binary program using that library.

Restriction 4.a

  1. Restrictions.The license granted in Section 3 above is expressly made subject to and limited by the following restrictions:

    a. You may distribute [...] the Work only under the terms of this License, and You must include a copy of, or the Uniform Resource Identifier for, this License with every copy [...] of the Work You distribute [...]. You may not offer or impose any terms on the Work that alter or restrict the terms of this License or the recipients' exercise of the rights granted hereunder. You may not sublicense the Work. You must keep intact all notices that refer to this License and to the disclaimer of warranties. You may not distribute [...] the Work with any technological measures that control access or use of the Work in a manner inconsistent with the terms of this License Agreement. The above applies to the Work as incorporated in a Collective Work, but this does not require the Collective Work apart from the Work itself to be made subject to the terms of this License. [...]

Provides for making Collective Works as long as the Work parts licensed under CC BY-SA 2.0 are properly identified and not sub licensed in any way. But those Collective Works are not required themselves to be made subject to the terms of CC BY-SA 2.0.

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    To be convincing you need to explain yourself. It is insufficient to simply say "restriction 4.a supplied to grant 3.a in definition 1.a...". You have not given any supporting explanation why these statements should be interprted to allow linking with code under a different license. – Brandin Sep 16 at 7:09
  • It's grant 3.a, not grant 4.a. I'll edit further later on. :) – 13042 Sep 16 at 7:12
  • Edited. But the same comment applies. Simply idenfying "restrictoin 4.a", "grand 5.a", "definition 1.a" is not convincing. You could show each one, explain why you think they allow what is proposed (in contrary with the popular advice "do not use CC for code"). – Brandin Sep 16 at 7:14
  • I know, I know. I'll edit further later on. Busy Monday! – 13042 Sep 16 at 7:17
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    Is the GPL then also weak copyleft, because you can create an aggregate work where not all parts need to be governed by the GPL license? – Bart van Ingen Schenau Sep 16 at 17:05

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