Question of SQL join on nested field of right table

[izual]

Hi, Community:
I defined a dim table(tblDim) with schema :

root

 |-- dim_nested_fields: ROW<`id` INT, `product_name` STRING>




and the part of SQL is : JOIN ... ON leftTable.`nested_field`.id = 
tblDim.`dim_nested_fields`.id.




which will throw an exception like:

Exception in thread "main" org.apache.flink.table.api.TableException: Temporal 
table join requires an equality condition on fields of table 
[MockTableSource(dim_nested_fields)].




Then I found these code snippets in UnitTest: FOR SYSTEM_TIME AS OF T.proctime 
AS D ON T.a + 1 = D.id + 2
Which will throw the same exception too.


In my opinion, these two are not the same case.
Why ` join ... on ... tblDim.`dim_nested_fields`.id ` failed, is it by design 
or a bug?
And what's the recommended way to solve this?


Thanks.