I think @Levi_Roth’s analysis is spot on. Jane Street ppxes
ultimately rely on ppxlib, whose version for v0.12 selects the
4.07 AST.
It is also true that the bleeding edge repository does use a
ppxlib version whose AST is 4.08, thus fixing the issue.
The only useful thing I can add is that we are currently in
the early stages of v0.13 (which will be based on what is
currently in the bleeding edge repository). The upcoming
version is expected to be fully compatible with 4.08 and 4.09.