Failure in Lambda return type inference

[Jesper Öqvist]

ExtendJ 8.0.1-204-ga9ab455 Java SE 8

Test:

// .result: COMPILE_PASS
import java.util.function.Function;

public class Test {
  public static void main(String[] args) {
    int res = map(1, e->e);
  }

  static <T, R> R map(T e, Function<T, R> f) {
    return f.apply(e);
  }
}

Expected result: no errors

Actual result: ExtendJ fails to infer the correct type for the generic method:

    [junit] Compilation failed when expected to pass:
    [junit] tests/jsr335/lambda/type_inf_06p/Test.java:7: error: can not assign variable res of type int a value of type java.lang.Object

[Jesper Öqvist]

Initial constraints produced in the current implementation:

• T :> Integer
• T = T
• R = R

These initial constraints are produced from just the map arguments. Since there exist equality constraints, the inference does not continue to looking at the return type. This prevents it from inferring that R should have type Integer.

[Jesper Öqvist]

This seems to be a case where the new type inference from Java 8 is needed. See JLS 8 §18.

Both the constraints from the map call and the lambda type inference constraints should be solved at the same time to find the solution in this case.