In a *shell* buffer, if I position the cursor after typing "pass"
and invoke M-: (shell-dynamic-completion-command),

this should yield a completion "passwd" if /bin is in exec-path.
However it errors out with

error: (wrong-number-of-arguments (3 . 4) 5)
  completion-in-region(484 486 #f(compiled-function (string pred action) #<bytecode -0x6e453bfcd314ab>) :exit-function #f(compiled-function (string finished) #<bytecode -0x9a98b1a3de78800>))
  apply(completion-in-region (484 486 #f(compiled-function (string pred action) #<bytecode -0x6e453bfcd314ab>) :exit-function #f(compiled-function (string finished) #<bytecode -0x9a98b1a3de78800>)))
  shell-dynamic-complete-command()
  eval((shell-dynamic-complete-command) t)
  eval-expression((shell-dynamic-complete-command) nil nil 127)
  funcall-interactively(eval-expression (shell-dynamic-complete-command) nil nil 127)
  call-interactively(eval-expression nil nil)
  command-execute(eval-expression)

The completion data functions shell-environment-variable-completion and
shell-command-completion include an :exit-function keyword value pair in
the result. But this cannot be passed on to completion-in-region.  (How
is this exit-function to be used at all?)

Commit e71564921ec760638105c910fdfae8b648724130 fixed similar issues in
comint.el, and probably missed shell.el during that pass


* lisp/shell.el (shell-dynamic-complete-command)
(shell-dynamic-complete-environment-variable): Correctly call
completion-in-region.