Lambda parameters do not use the GLB of the inferred parameter type

[Jesper Öqvist]

ExtendJ 8.0.1-139-gb1294d4 Java SE 8

A lambda expression with an inferred type should use the greatest lower bound (GLB) of the parameter type for the lambda parameter type. For example:

import java.util.stream.Stream;

public class Test {
  void m(Stream<String> stream) {
    stream.map(s -> s.length());
  }
}

The type of the lambda expression above is Function<? super String, R> and the type of the parameter s should be the GLB, i.e. String, however ExtendJ assigns the type ? super String to s, as evidenced by the error message:

Test.java:8: error: no method named length() in wildcards.? super java.lang.String matches.

[Jesper Öqvist]

The Java specification seems to describe the type of a lambda expression to be the non-wildcard parameterization of the target function type.

See JLS 8 §9.9

[Jesper Öqvist]

Here is another test case showing the same error more clearly:

import java.util.function.Function;

public class Test {
  Function<? super String, Integer> fun = s -> s.length();
}

[Jesper Öqvist]

Improve function type inference

This improves wildcard type handling in function type inference.

fixes #181 (bitbucket)

→ <<cset e8ccc5411510>>