wait -n ignores invalid pids when valid pids are listed

[Beer, Mathis]

Given a background process that has exited before the script got to wait -n:

function foo() { return 1; }
foo & FOO_PID=$!

function bar() { sleep 1; }
bar & BAR_PID=$!

sleep 0.1
# should exit with 127 since FOO_PID is non-existent now
wait -n $FOO_PID $BAR_PID

Then wait -n will wait on BAR_PID to exit, despite the fact that FOO_PID is 
invalid. (Tested with 5.0.17.)

Shouldn't it immediately exit with 127, as the docs suggest, since one id 
specifies a non-existent process?

The way it currently works makes it hard to make a script that waits for one 
of a set of background processes to exit (ie. that fails if one of a set of 
background processes fails), since any error that happens before the script 
reaches the wait will simply be ignored.