|: =(hang)

[Oliver Kiddle <okiddle@xxxxxxxxxxx>]

The following hangs the shell which seems like a bug to me:
  zsh -f
  % echo hi | echo =(notfound)
  zsh: command not found: notfound
kill -9 is needed at this point.

Putting the not found command in the other place also hangs things:
  % notfound | echo =(echo hi)
  zsh: command not found: notfound
  /tmp/zshWEjDri

Two command substitutions but no pipe seems to be fine, e.g : =(notfound) =(:)

This behaviour isn't new. And as it isn't a regression, I wouldn't hold
up 5.5 or anything.

Oliver