SC2001 should not be recommended when shebang is /bin/ksh

[mbbh]

For bugs

• SC2001:
• My shellcheck version (shellcheck --version or "online"): 0.9.0
• [X] The rule's wiki page does not already cover this (e.g. https://shellcheck.net/wiki/SC2086)
• [X] 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:

#!/bin/ksh
x="foo"
foo=$(echo "$x" | sed -e 's,foo,bar,')
echo "$foo"

Here's what shellcheck currently says:

^-- SC2001 (style): See if you can use ${variable//search/replace} instead.

Here's what I wanted or expected to see:

Shellcheck should not be suggesting this as this is a bash/zsh feature and not present in ksh

in ksh using ${a//b/c} or ${a//b/c} will produce 'bad substitution' when used in the way suggested by shellcheck.