Github user JoshRosen commented on the issue:
https://github.com/apache/spark/pull/15571
Yep, this new MiMa inspection was added in
https://github.com/typesafehub/migration-manager/issues/119:
> When a new abstract class/trait with abstract members is added to the
hierarchy of an existing class/trait, a binary incompatibility must be reported
if the abstract members added by the new type did not exist in the hierarchy of
the existing class/trait.
Looking at our particular error, it appears that this was introduced by
https://github.com/apache/spark/commit/61ef74f2272faa7ce8f2badc7e00039908e3551f,
which added a new `HasAggregationDepth` trait and then mixed it into other
traits.
It looks like `HasAggregationDepth` actually defines `final`
implementations for al of its methods:
```scala
/**
* Trait for shared param aggregationDepth (default: 2).
*/
private[ml] trait HasAggregationDepth extends Params {
/**
* Param for suggested depth for treeAggregate (>= 2).
* @group param
*/
final val aggregationDepth: IntParam = new IntParam(this,
"aggregationDepth", "suggested depth for treeAggregate (>= 2)",
ParamValidators.gtEq(2))
setDefault(aggregationDepth, 2)
/** @group getParam */
final def getAggregationDepth: Int = $(aggregationDepth)
}
```
I think that a trait with fields / implementations compiles down to both an
interface and an abstract class in Scala 2.10 / 2.11, so I imagine that this
isn't actually okay. This is confirmed by one of the test cases in MiMa:
https://github.com/dotta/migration-manager/tree/54d54a8d1d7f0da8909c860e734bc1335ddae4c4/reporter/functional-tests/src/test/trait-extending-new-trait-with-concrete-method-nok
@dotta, to clarify the wording in your commit to MiMa, is it accurate to
replace "abstract members" by just "members" and say that adding any members to
a trait, abstract or concrete, can introduce a binary incompatibility if it
wasn't already presence in subclasses's existing hierarchy?
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