trap on command exit not working unless used in a function or sub-shell

Question

Am experimenting a bit with trap on invalid command return code using a sample code

#!/bin/bash

# Exit on error
trap 'echo 'exiting..';exit' ERR
set -e

h=1
b=$((h+)) # <----- command causing the error
echo $?
echo $b
echo "end"

Am not able to cause the trap to occur even though there is an offending instruction. But the same if I run in a sub-shell as

function junk() {
h=1
b=$((h+))
echo "Exit code:$?"
echo $b
echo "end"
echo "Hello"
}

junk

(or) running the entire instructions in a sub-shell as

(h=1
b=$((h+))
echo "Exit code:$?"
echo $b
echo "end"
echo "Hello"
)

am able to catch the command failure and the EXIT catches the trap and prints the message accordingly.

Am aware of the usage of set -e

-e  errexit When set, the shell exits when a simple command in a command list exits
    non-zero (FALSE). This is not done in situations, where the exit code is already checked 
    (if, while, until, ||, &&)

but I can't find a proper reference where it says happens only on a sub-shell or a function or something similar.

Let me know if I am missing something basic here.

Answer 1

What you are missing is that set -e and sigspec ERR both apply to command execution (commands that exit with a non-zero value). What you have here with b=$((h+)) is a parsing error. The command is not executed because it is not understood.

Why does it work in a function or sub-shell? Because this parsing error makes the containing script (be it function or a sub-shell) fail as a whole. In other words, this is not b=$((h+)) that triggers your trap (there is no trap set in your sub-shells), this is the failure of the call to junk or to your subshell.