Plus sign in a link - is it allowed?

In w:Help:Link it states clearly that "in internal and
interwiki
link style, a plus sign in a page name is not allowed, the
HTML
and hence the rendered page just shows the wikitext, e.g.
a+b".

However, looking at that page live on Wikipedia, the plus
sign
*is* shown in the link.

On my wiki (1.5.6) [[a+b]] is rendered as the wikitext.

I couldn't find any release notes that describes this
change.
Is it a Wikipedia customisation? Will upgrading MediaWiki
give
me this feature? Is it an actual feature, or just some crazy
undocumented behaviour that I should steer clear of?

I've been using "CPlusPlus" when I mean
"C++", but I'd love to
change if someone can clear this up for me. I'll even
update
the documentation on Wikipedia out of gratitude 

cheers,
Matt Curtis
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Matt Curtis wrote:
> In w:Help:Link it states clearly that "in
internal and interwiki
> link style, a plus sign in a page name is not allowed,
the HTML
> and hence the rendered page just shows the wikitext,
e.g. a+b".

In MediaWiki 1.6, you can optionally add to the set of
allowed title characters.

This breaks backwards compatibility with some very old
Wikipedia URLs from 2002,
so is somewhat controversial. (eg if you follow one of those
a very old links
that has been recorded, linked, or referenced somewhere it
no longer brings you
to the right page.)

-- brion vibber (brion  pobox.com)

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