Why does rstan 2.19.2 clone stan/lang/grammars directory?

What’s the rationale for v2.19.2 to have entire stan lang’s grammars directory cloned? In v2.18.x these src’s are from StanHeaders and rstan just have to #include them. Which path does rstan 2.19.2 actually use in compilation, its own grammars or StanHeaders’?

The StanHeaders ones are copied from 2.19 into rstan and then used. This was necessary to avoid using the ones that came with, which were modified to work with rstan 2.18 to restore backward compatibility. For 2.20, hopefully we won’t have to do (as) much.