presto-main/src/main/java/io/prestosql/operator/scalar/CastFromUnknownOperator.java/CastFromUnknownOperator/specialize
class CastFromUnknownOperator:    @Override
    public ScalarFunctionImplementation specialize(BoundVariables boundVariables, int arity, Metadata metadata)
    {
        Type toType = boundVariables.getTypeVariable("E");
        MethodHandle methodHandle = METHOD_HANDLE_NON_NULL.asType(METHOD_HANDLE_NON_NULL.type().changeReturnType(toType.getJavaType()));
        return new ScalarFunctionImplementation(
                false,
                ImmutableList.of(valueTypeArgumentProperty(RETURN_NULL_ON_NULL)),
                methodHandle);
    }