Curved cors anglais and oboes da caccia were usually made from maple with straight joints to start with (and bored), then wedges were cut out in several places along the length of the joints, the joints bent and glued back together, the outside refinished and then covered with leather to hide the glued joints. Some may have been bent by steaming and clamping the joints in a jig or form until they cooled so the curve was permanently locked in.
Here's some x-ray images of curved cors (and curved basset horns):