I was experiencing this problem when using golang's elasticsearch client "elastic".
*json.RawMessage is a type of []byte.If I not write []byte() to cast type, it said cannot use 'resp.Source' (type *json.RawMessage) as type []byte.
The environment is go 1.10.3 and elastic 6.x
A *json.RawMesasge is not a []byte. It's a pointer.
Dereference the pointer to convert to a slice of bytes:
byteSlice := []byte(*pointerToRawMessage)