False positive on SC2191 when using += assignment operator

[Xophmeister]

For bugs

• Rule Id (if any, e.g. SC1000): SC2191
• My shellcheck version (shellcheck --version or "online"): 0.10.0
• 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:

#!/usr/bin/bash

x=(foo bar [1]+=quux)
echo "${x[@]}"

Here's what shellcheck currently says:

In - line 3:
x=(foo bar [1]+=quux)
               ^---^ SC2191 (warning): The = here is literal. To assign by index, use ( [index]=value ) with no spaces. To keep as literal, quote it.

Did you mean:
x=(foo bar [1]+"=quux")

For more information:
  https://www.shellcheck.net/wiki/SC2191 -- The = here is literal. To assign ...

Here's what I wanted or expected to see:

This shouldn't trigger SC2191.

Note: Only the += operator works in this context, even with typeset -i; ++ and -= trigger syntax errors in Bash, whereas --, *= and /= get interpreted as literals.