I did the experiments and the most flexible declaration that worked with the
lambda is the following:
```
<K1, V1> KStream<K1, V1> transform(final TransformerSupplier<? super K, ? super
V, ? extends KeyValue<K1, V1>> transformerSupplier,
final String... stateStoreNames);
```
With this declaration, the `Transformer` may not return keys and values of
subclasses of the key type and the value type of the result stream. Thus, the
examples with `stream4` and `stream5` in @vvcephei 's experiments above do not
compile with this declaration.
If nobody objects, I will proceed with the declaration above.
Removing only the `? extends` in front of `KeyValue` instead of removing `?
extends` from `K1` and `V1` does also not compile when using lambdas.
FYI, I created a minimal example of this rather strange compiler behavior and
posted a question on stackoverflow [1].
[1]
https://stackoverflow.com/questions/52445222/java-using-lambda-impedes-inference-of-type-variable
[ Full content available at: https://github.com/apache/kafka/pull/5273 ]
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