`expr "$x" : '^$y"` is not reported as non-posix

[cD1rtX3]

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

#!/bin/sh
expr "X$1" : '^Xspecial'

Here's what shellcheck currently says:

Nothing.

Here's what I wanted or expected to see:

Something like “expr already only matches at the start of the string, so the ^ anchor is undefined for it in POSIX sh”.