command substitution: zsh waits until command exits

[Vincent Lefevre <vincent@xxxxxxxxxx>]

The zshexpn(1) man page says:

COMMAND SUBSTITUTION
  A command enclosed in parentheses  preceded  by  a  dollar  sign,  like
  `$(...)',  or quoted with grave accents, like ``...`', is replaced with
  its standard output, with any trailing newlines deleted.  If  the  sub-
  stitution  is  not enclosed in double quotes, the output is broken into
  words using the IFS parameter.  The substitution `$(cat  foo)'  may  be
  replaced  by  the  equivalent but faster `$(<foo)'.  In either case, if
  the option GLOB_SUBST is set, the output is eligible for filename  gen-
  eration.

but doesn't say when zsh does the substitution and runs the main
command (it should be clarified). For instance, with the following
command

  echo $(echo foo; exec >&-; sleep 2)

zsh waits for 2 seconds before outputting 'foo'. However, since the
standard output fd of the substituted command has been closed, I assume
that zsh should be able to replace $(echo foo; exec >&-; sleep 2) by
'foo' and run the main command before the command inside $(...) exits.
Is this a bug?

Otherwise, is there a way to do what I wish (without temporary files)?
More precisely, I want to do something like:

  some_command $(xterm -e 'tty >&3; exec 3>&-; sleep 999999' 3>&1)

so that some_command can write data to the xterm (in addition to the
normal output).

-- 
Vincent Lefèvre <vincent@xxxxxxxxxx> - Web: <http://www.vinc17.org/>
100% accessible validated (X)HTML - Blog: <http://www.vinc17.org/blog/>
Work: CR INRIA - computer arithmetic / Arenaire project (LIP, ENS-Lyon)