Github user koertkuipers commented on the issue:
https://github.com/apache/spark/pull/15979
this means anything that uses an encoder can no longer use Option[_ <:
Product].
encoders are not just used for the top level Dataset creation.
Dataset.groupByKey[K] requires an encoder for K.
KeyValueGroupedDataset.mapValues[W] requires an encoder for V
Aggregator[A, B, C] requires encoders for B and C
none of these always create top level row objects (for which this pullreq
creates the restriction that they cannot be null).
for an aggregator it is sometimes the case.
```dataset.select(aggregator)``` does create a top level row object, but
```dataset.groupByKey(...).agg(aggregator)``` does not.
so i am not sure it makes sense to put this restriction on the encoder. it
seems to belong on the dataset.
another example of something that won't work anymore:
```
val x: Dataset[String, Option[(String, String)]] = ...
x.groupByKey(_._1).mapValues(_._2).agg(someAgg)
```
in this case the mapValues requires``` Encoder[Option[(String, String)]]```
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