PLS-00318：type "string" is malformed because it is a non-REF mutually recursive type

错误原因：A type-declaration such as: -- a non-REF recursive type, for example: type t is record (a t); or -- non-REF mutually dependent types, for example: type t1; type t2 is record (a t1); type t1 is record (a t2); was performed. In these examples, the type is malformed because it participates in a mutually recursive non-REF relationship with itself and possibly other types.