[Help-bash] Errorneously flagged undefined variable with here string redirected to bash-allocated file descriptor when followed by a command?

[gima+ml . help-bash]

Hello,

I've encountered an error where bash says that a variable is undefined 
when I'm accessing it, even though to my understanding it's supposed to 
be defined at that point. I'm asking if I'm misunderstanding something, 
or should this be treated as a bug.

I'm using the GNU bash, version 4.3.30(1)-release (x86_64-pc-linux-gnu), 
and regarding this problem, the following relevant features:

 * set -o nounset /(accessing unset variables causes error)/
 * here strings redirection
 * bash-allocated file descriptor from redirection, whose number is
   stored to a variable
 * command execution with the variable as an argument

Below are examples that demonstrate the error.


       Common input file for both of the examples below:

> -- showarg.sh --
>
> #!/usr/bin/bash
> set -vuex -o pipefail
> echo ${@@A} ## SHOW ARGUMENTS
>
> cat "$1" ## DISPLAY GIVEN FILE'S CONTENT


     Example code that DOESN'T work:


       Input:

> -- test.sh --
>
> #!/usr/bin/bash
> set -vuex -o pipefail
>
> ## CREATE FILE DESCRIPTOR FROM GIVEN STRING
> ## AND CALL SCRIPT THAT DISPLAYS THE CONTENT OF A GIVEN FILE
> {FD}<<<'hello' \
>   ./showarg.sh "/dev/fd/$FD"


       Output:

> $ ./test.sh
>
> ## CREATE FILE DESCRIPTOR FROM GIVEN STRING
> ## AND CALL SCRIPT THAT DISPLAYS THE CONTENT OF A GIVEN FILE
> {FD}<<<'hello' \
>   ./showarg.sh "/dev/fd/$FD"
> ./test.sh: line 6: FD: unbound variable
To my understanding the code should work, because the following works: 
555<<<'hello' ./showarg.sh "/dev/fd/555"


     Example code that WORKS:


       Input:

> -- test2.sh --
>
> #!/usr/bin/bash
> set -vuex -o pipefail
>
> exec {FD}<<<'hello' ## CREATE FILE DESCRIPTOR FROM GIVEN STRING
>
> ./showarg.sh "/dev/fd/$FD" ## CALL SCRIPT THAT DISPLAYS THE CONTENT OF 
> A GIVEN FILE


       Output:

> $ ./test2.sh
>
> exec {FD}<<<'hello' ## CREATE FILE DESCRIPTOR FROM GIVEN STRING
> + exec
>
> ./showarg.sh "/dev/fd/$FD" ## CALL SCRIPT THAT DISPLAYS THE CONTENT OF 
> A GIVEN FILE
> + ./showarg.sh /dev/fd/10
> echo ${@@A} ## SHOW ARGUMENTS
> + echo set -- ''\''/dev/fd/10'\'''
> set -- '/dev/fd/10'
>
> cat $1 ## DISPLAY GIVEN FILE'S CONTENT
> + cat /dev/fd/10
> hello


- gima