use abstract implementation fields only in left-mode when detecting operator overloads (closes #2130)

typer.ml

@@ -1749,11 +1749,15 @@ let rec type_binop ctx op e1 e2 is_assign_op p =
 			| (o,cf) :: ops when is_assign_op && o = OpAssignOp(op) || o == op ->
 				(match follow (monomorphs cf.cf_params cf.cf_type) with
 				| TFun([(_,_,t1);(_,_,t2)],r) ->
-					let t1,t2 = if left || Meta.has Meta.Commutative cf.cf_meta then t1,t2 else t2,t1 in
-					if type_iseq t t2 && (if Meta.has Meta.Impl cf.cf_meta then type_iseq (apply_params a.a_types pl a.a_this) t1 else type_iseq (TAbstract(a,pl)) t1) then begin
-						if not (can_access ctx c cf true) then display_error ctx ("Cannot access operator function " ^ (s_type_path a.a_path) ^ "." ^ cf.cf_name) p;
-						cf,r,o = OpAssignOp(op),Meta.has Meta.Commutative cf.cf_meta
-					end else loop ops
+					let impl = Meta.has Meta.Impl cf.cf_meta in
+					(* implementation fields can only be used in left mode (issue #2130) *)
+					if impl && not left then loop ops else begin
+						let t1,t2 = if left || Meta.has Meta.Commutative cf.cf_meta then t1,t2 else t2,t1 in
+						if type_iseq t t2 && (if impl then type_iseq (apply_params a.a_types pl a.a_this) t1 else type_iseq (TAbstract(a,pl)) t1) then begin
+							if not (can_access ctx c cf true) then display_error ctx ("Cannot access operator function " ^ (s_type_path a.a_path) ^ "." ^ cf.cf_name) p;
+							cf,r,o = OpAssignOp(op),Meta.has Meta.Commutative cf.cf_meta
+						end else loop ops
+					end;
 				| _ -> loop ops)
 			| _ :: ops ->
 				loop ops