Description Zorzella Mising name 2019-02-25 14:09:57 EST

The Snippet below is a stripped-down version of a piece of code I came across that javac compiles, but Eclipse shows a type mismatch error, in this case:

Type mismatch: cannot convert from Snippet.Outer<Object> to Snippet.Outer<Snippet.Inner<T>>

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package typebug;

import java.io.Serializable;
import java.util.AbstractCollection;
import java.util.List;
import java.util.Optional;
import java.util.RandomAccess;
import java.util.concurrent.Executor;
import java.util.concurrent.Future;
import java.util.function.Function;
import java.util.stream.Collector;
import java.util.stream.Stream;

public class Snippet {

  public static <T> void callTransform(
      Outer<? extends Optional<? extends T>> outerInput) {
    Outer<Inner<T>> foo = transform(outerInput, input -> stream(input).collect(toInner()));
  }
  
  public interface Outer<V> {}

  public interface Inner<E> {}

  public static <E> Collector<E, ?, Inner<E>> toInner() {
    return null;
  }

  public static <T> Stream<T> stream(Optional<T> optional) {
    return null;
  }

  public static <I, O> Outer<O> transform(
      Outer<I> input,
      Function<? super I, ? extends O> function) {
    return null;
  }
}