Assigning to BASHPID fails silently

[Martijn Dekker]

bash 4.4.0 (I did not investigate other versions) does not produce an
error message if you try to assign something to the BASHPID readonly
using either arithmetic or normal assignment. Other readonlies produce a
message on an assignment attempt. BASHPID seems to be an exception.

<background type="superfluous">
Particularly annoying is that a non-interactive shell will exit silently
on a mistaken attempt to assign to BASHPID. I was making a bash function
to determine if we're in a subshell. Because of the lack of error
message, it took some doing to find my typo in
        return "$((BASHPID = $$))"
(of course, comparison is '==' and not '='). Not even 'set -x' will give
a clue; it exits before producing relevant tracing output.
</background>

Proof below. (The exit status of the previous command is included in the
prompt in the output below.)

[0]$ UID=0
bash: UID: readonly variable
[1]$ BASHPID=0
[1]$ ((UID=0))
bash: UID: readonly variable
[1]$ ((BASHPID=0))
[1]$

Thanks,

- Martijn