Re: eqn delimiters break conditionals

[Ingo Schwarze <schwarze@usta.de>]

Hi,

Nab wrote on Wed, Sep 08, 2021 at 03:24:48PM +0200:

> Consider the following file:
> -- >8 --
> .EQ
> delim %%
> .EN
> .if 0 A % B % C
> .if 1 D % E % F
> -- >8 --
[...]

> However, mandoc (1.14.4-1 and latest CVS as of an hour ago) does not:
> -- >8 --
> $ ./mandoc < a.1 | grep -v '^$'
> ()                                                                          ()
>  B C D E F
>                                                                             ()
> -- >8 --
[..]
> This smells like a bug to me, for reasons I assume obvious.

Yes, that is a bug, and it was so far unknown, too.

The reason for the bug is that roff.c implements eqn(7) delim
by replacing delimited equations with .EQ/.EN blocks on the fly,
and then they are no longer on the conditional line.

Fixing it requires to either implement delim in a completely
different way, which would cause very major code changes.

Or the delim handling could possibly be moved after the handling
of conditionals in roff_parseln().  But the code in that function
is very prone to regressions.

Consequently, i won't attempt fixing this bug before the upcoming
release, the risk of breaking the release in a way that might be
even worse is too high, and besides, this causes too much effort
to be resolved before release.

Instead, i'm adding an entry to the TODO file:

- eqn(7) delimiters cause conditional lines to misbehave
  nabijaczleweli 8 Sep 2021 15:24:48 +0200
  loc *  exist ***  algo ***  size *  imp *

Thanks for reporting!
  Ingo
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