presto-main/src/main/java/io/prestosql/operator/scalar/CastFromUnknownOperator.java/CastFromUnknownOperator/CastFromUnknownOperator
class CastFromUnknownOperator:    public CastFromUnknownOperator()
    {
        super(CAST,
                ImmutableList.of(typeVariable("E")),
                ImmutableList.of(),
                new TypeSignature("E"),
                ImmutableList.of(UNKNOWN.getTypeSignature()),
                false);
    }