In RdrHsSyn.checkAPat the processing for ExprWithTySig is defined as

   ExprWithTySig e t _ -> do e <- checkLPat msg e
                             -- Pattern signatures are parsed as sigtypes,
                             -- but they aren't explicit forall points.  Hence
                             -- we have to remove the implicit forall here.
                             let t' = case t of
                                        L _ (HsForAllTy Implicit _ _
                                             (L _ []) ty) -> ty
                                        other -> other
                             return (SigPatIn e (mkHsWithBndrs t'))

The t' variable ends up losing its original SrcSpan in the first case
branch. This results in annotations becoming detached from the AST.

Reviewed By: austin

Differential Revision: https://phabricator.haskell.org/D823

GHC Trac Issues: #10255