virtual/alternative B-D (was Re: libtiff5 transition)

[Thorsten Glaser <t.glaser@tarent.de>]

On Thu, 5 Dec 2013, Jay Berkenbilt wrote:
>  * If your package build-depends on libtiff5-dev, you don't HAVE to do
>    anything, but you may be helping yourself in the future if you change
>    the build dependency to libtiff-dev (>> 4.0.3-6~).

Uhm, I have a rather general question here.

libtiff-dev is a virtual package (it’s only provided by others).
Asides from the issue of virtual packages and versioning, I’ve
had ftpmasters REJECT a package of mine in NEW when it had a
Build-Depends on a virtual package (libncurses-dev, which I
had to change to “libncurses5-dev | libncurses-dev” first,
and “libtinfo-dev | libncurses-dev” now that libtinfo is in
the archive).

I thought we’re supposed to not build-depend on virtual
packages, especially not as first alternative?

On the other hand, I totally do not understand *how* this
can possibly work, given that buildd only uses the first
alternative in Build-Depends anyway. (So basically, why
(except for backports) bother having alternative B-D in
packages at all?)

I fail to see the reason behind all this; can someone
explain why both the “don’t depend on virtual packages”
rule seems to be in effect (although apparently only
for NEW, never checked later) and buildd does not use
alternatives (never got that one)?

Thanks,
//mirabilos
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