Variable assignments without brackets inside `$(( ... ))` operations confuse Shellcheck

[Winterhuman]

For bugs with existing features

• Rule Id: Various, see snippet descriptions.
• My shellcheck version: 0.11.0
• The rule's wiki page does not already cover this (e.g. https://shellcheck.net/wiki/SC2086)
• I tried on https://www.shellcheck.net/ and verified that this is still a problem on the latest commit

Here's a snippet or screenshot that shows the problem:

Arithmetic operations, $(( ... )), can contain variable assignments (e.g. var = 1); however, this breaks shellcheck's parsing and causes various unrelated errors if the assignments aren't wrapped in brackets (e.g. (var = 1)).

#!/bin/sh

arg="$1"	var2=""
var="$(( arg < 2 ? var2 = 1 : 0 ))"

echo "$var : $var2"

What this script prints when executed by sh, ksh, bash, or dash (no difference if POSIXLY_CORRECT=1 is set):

$ ./script.sh 1
1 : 1
$ ./script.sh 2
0 : 

Shellcheck suggests the following SCs for the script above: 1102, 2034, and 2210.

---

#!/bin/sh

arg="$1"	var2=""
var="$((
	arg < 2
	? var2 = 1
	: 0
))"

echo "$var : $var2"

What this script prints when executed by sh, ksh, bash, or dash (no difference if POSIXLY_CORRECT=1 is set):

$ ./script.sh 1
1 : 1
$ ./script.sh 2
0 : 

When the operations is written over multiple lines, Shellcheck additionally suggests SC2211 with the previously listed SCs.

If (var2 = 1) is used instead of var2 = 1, no SCs are suggested when using either version of the operation.

Here's what shellcheck currently says:

See above.

Here's what I wanted or expected to see:

Shellcheck should handle arithmetic operations containing bracket-less variable assignments which, as far as I can tell, is valid syntax for POSIX shell, Korn shell, Bash, and Dash.

[Winterhuman]

Correction: If the operation is something like $(( x > 1 ? y = 1 : z = 1 )), then this isn't understood by Bash; brackets are always required for variable assignments within the : line.

It's kind of weird that only the second assignment needs brackets, so maybe it'd be better to suggest always using brackets for variable assignments inside $(( )). Regardless, Shellcheck currently prints unrelated SCs when this is encountered.