`is-macos() [[ $OSTYPE == darwin* ]]` is valid Bash

[ingydotnet]

I just realized that:

is-macos() [[ $OSTYPE == darwin* ]]

Is a valid Bash (3.2+) function definition.

A cleaner alternative to:

is-macos() ( [[ $OSTYPE == darwin* ]] )
or:
is-macos() { [[ $OSTYPE == darwin* ]]; }

shellcheck 0.8.0 gives:

is-macos() [[ $OSTYPE == darwin* ]]
           ^-- SC1064 (error): Expected a { to open the function definition.
           ^-- SC1072 (error):  Fix any mentioned problems and try again.

Also valid Bash (3.2+):

big-num() (( $1 > 9 ))

Fortunately I can disable these for now.