It is a configuration option: $wgNamespacesWithSubpages.
For WMF wikis it is set to allow subpages for several namespaces, including talk, but not the main namespace.

Hi Michael,

Thanks for your bug report and helping the MediaWiki software. The situation you describe is not a real bug. By default in the software neither the Article or Article talk page have subpage-behaviour enabled (because articles can often contain slashes, like your example shows), thus this bug does not exist or occur in the software itself.

Wikipedia, however, has configured their wiki with subpages specifically enabled in the Article talk namespace. The reason for this was probably to allow having subpages like /Talk_archive.

I'm marking this as INVALID because it's not a bug but a requested configuration by the Wikipedia community. If you believe subpages should be disabled in talk pages of articles on Wikipedia, please make a proposal at the Wikipedia village pump [1].

[1] http://en.wikipedia.org/wiki/Wikipedia:Village_pump

Ah, looks like I misinterpreted the config-file. Looks like wgNamespacesWithSubpages actually contains the article talk space by default.

Err, this is quite clearly a bug. There's overlap between a legitimate title character ("/") and the subpage delimiter ("/"). I'm fairly sure this is a duplicate of a pre-existing bug, though, so I won't re-open.

Another column such as page_is_subpage would fix this fairly easily.

(In reply to comment #4)

Err, this is quite clearly a bug. There's overlap between a legitimate title
character ("/") and the subpage delimiter ("/"). I'm fairly sure this is a
duplicate of a pre-existing bug, though, so I won't re-open.

Another column such as page_is_subpage would fix this fairly easily.

That's true, in a way. But on the other hand:

• the content page tab does correctly link to [[L/poly]] from [[Talk:L/poly]]
• both [[L]] and [[Talk:L]] exist, so it may be a valid sub page. Just like [[Talk:Foo/Archive]] would be.

Ofcourse there could be a solution where page hiearchy is cut loose from the title all together (ie. title of a subpage would be "Archive" with NS_TALK / "Foo" as parent (or a page id). But without a very major overhaul, this is the intended behaviour. One could disable subpages in the main namespace's talk namespace. That may not be such a crazy idea actually. How common are valid subpages in NS_TALK ?

But without a very major overhaul, this is the
intended behaviour. One could disable subpages in the main namespace's talk
namespace. That may not be such a crazy idea actually. How common are valid
subpages in NS_TALK ?

Very common when you consider talk page archives. In fact I'd consider that one of the main use cases of said feature.