pjfanning opened a new pull request, #3719:
URL: https://github.com/apache/pekko/pull/3719
### Motivation
Java generics are invariant, so javadsl operators that take a `Graph` reject
graphs of a narrower or wider element type even when the call is type safe:
concatenating a `Source<Integer>` onto a `Source<Number>` needs
`Source.upcast`, and a pre-typed `Sink<Object, ...>` or `Flow<Object, ...>`
cannot be attached to a `Source<String>`. Functions returning `Optional`,
`Iterable` or `Pair` are also still invariant in their element types after
#3717 widened the function parameters themselves. These are items 1 and 2 from
the follow-up comment on #3712.
### Modification
- Widen `Graph` parameters across the stream javadsl, including graphs
inside function results, `Iterable`s and `CompletionStage`s:
- `Graph[SourceShape[T], M]` → `Graph[? <: SourceShape[? <: T], M]`
(`concat`, `merge`, `zip*`, `interleave*`, `orElse`, `prepend*`,
`flatMapConcat`, `recoverWith`, …)
- `Graph[SinkShape[T], M]` → `Graph[? <: SinkShape[? >: T], M]` (`to`,
`toMat`, `runWith`, `alsoTo*`, `wireTap*`, `divertTo*`, …)
- `Graph[FlowShape[I, O], M]` → `Graph[? <: FlowShape[? >: I, O], M]`
(`via`, `viaMat`, `join`, …)
- The outer `? <:` is required: `Graph` itself is invariant in Java, so
without it a `Graph<SourceShape<Integer>, M>` would not match even an exact
`SourceShape<? extends Integer>`.
- Widen `Optional[T]`, `java.lang.Iterable[T]` and `Pair[A, B]` where they
are the result of a function parameter (`mapOption`, `mapConcat`,
`statefulMap`, `unfold*`, `RetryFlow`, …).
- Internals:
- `sourceGraph`/`sinkGraph`/`flowGraph` helpers narrow the widened graphs
before delegating to scaladsl. Scala 3.3 does not treat e.g. `SourceShape[? <:
T]` as conforming to `SourceShape[T]` even though the shapes are variant;
erasure is identical, so the casts are safe.
- `optionalToScala`/`iterableToScala` helpers, because Scala 2 cannot
apply the converters to the nested existential types; a cast in `unfold` where
the internal stage takes an invariant function.
- `RunnableGraphSpec`: give the javadsl round trip a concrete element type.
Scala 2 cannot infer `Sink.ignore()` against `Graph[? <: SinkShape[? >:
Nothing], ?]`; with any concrete element type, Scala callers using generic
factories (`Sink.ignore()`, `Sink.seq`, `Source.empty()`, `Flow.create()`)
still infer on 2.13 and 3.3.
- Add MiMa excludes (`IncompatibleSignatureProblem` only) for the generic
signature changes.
### Result
Java callers can combine sources of a subtype and attach flows and sinks of
a supertype without `upcast` or casts, and functions may return an
`Optional`/`Iterable`/`Pair` of a subtype. Erasure is unchanged, so the change
is binary compatible.
### Tests
- `sbt "stream/compile" "++3.3.8 stream/compile"` - passed
- `sbt "stream-tests/Test/compile"` (existing Java and Scala callers plus
new tests) - passed
- `sbt "stream/mimaReportBinaryIssues"` - passed with the added excludes
- New directional tests (`mustAcceptGraphsOfNarrowerAndWiderElementTypes`,
`mustAcceptFunctionsReturningOptionalsAndIterablesOfSubtypes` in `SourceTest`)
not run locally - left to CI, along with the other modules' test compilation
### References
Refs #3712, #3717
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