DeriveGeneric: look up datacon fixities using getDataConFixityFun
Previously, DeriveGeneric
would look up the fixity of a data constructor
using getFixityEnv
, but this is subtly incorrect for data constructors
defined in external modules. This sort of situation can happen with
StandaloneDeriving
, as noticed in #20994 (closed). In fact, the same bug has occurred
in the past in #9830 (closed), and while that bug was fixed for deriving Read
and
deriving Show
, the fix was never extended to DeriveGeneric
due to an
oversight. This patch corrects that oversight.
Fixes #20994 (closed).