dash bug with string expansion and assignment of default values

Noticed when compiling OpenFOAM-v2006 modules with ubuntu bionic (18.04) that the module prefix is not being correctly set. Seems to be a weird bug in the dash shell expansions.

Starting from any non-root directory

echo "$PWD"
-> /home/user/openfoam/BuildEnv/scratch

dash 0.5.8-2.10 (bionic) shows this

unset test1
echo "${test1:=${PWD%/*}}"
-> /home/user/openfoam/BuildEnv/scratch  <-- WHAT?? Not what we expected

The additional quotes cause incorrect expansion. Whereas

unset test1
echo ${test1:=${PWD%/*}}
-> /home/user/openfoam/BuildEnv  <-- Expected

• bash and dash 0.5.10.2-6 (focal) work as expected, with/without quotes
• quoted expansion with :- works as expected.

Possible fixes?

• https://git.kernel.org/pub/scm/utils/dash/dash.git/commit/?id=48ca00863af909461d1372998bb90549f27abaaf
• https://git.kernel.org/pub/scm/utils/dash/dash.git/commit/?id=a29e9a1738a4e7040211842f3f3d90e172fa58ce
• https://git.kernel.org/pub/scm/utils/dash/dash.git/commit/?id=878514712c5d21f675c45d99d2f8a04098ea4a19

Possible workarounds

• use unqoted version, but risk fixing it on the next shellcheck
• explicit if [ -z VAR ]; then ...; fi construct
• with dirname as : "${VAR:=$(dirname xxx)}