Right now, running go test -p 1 ./... results in the following failure (on go 1.16.2):

--- FAIL: TestEncodingPackTypeInvalid (0.00s)
    encoding_test.go:54: The packing function incorrectly succeeds for a non-packable value

This looks to be caused by ioutil.Discard being a valid value for packing. As of Go 1.16, this value was changed to be io.Discard which is just an empty struct (which is a valid value for packing).