[GROOVY-10913] Type inference yields a wrong type even if the type of the associated variable is given

• Type: Bug
• Status: Open
• Priority: Minor
• Resolution: Unresolved
• Component/s: Static Type Checker
• Assignee: Eric Milles
• Reporter: Thodoris Sotiropoulos
• Created: 22/Jan/23 11:12
• Updated: 15/Apr/23 16:03

Description

The following code demonstrates a limitation of type inference in a real-world example.
I have the following program

import java.util.*;
import java.util.stream.*;

class Test {
    void test() {
      Stream<Number> x = Arrays.stream(new Integer[]{1}).flatMap((y) ->
        Arrays.stream(new Long[]{(long) 1}));
    }
}

Actual behavior

org.codehaus.groovy.control.MultipleCompilationErrorsException: startup failed:
test.groovy: 6: [Static type checking] - Incompatible generic argument types. Cannot assign java.util.stream.Stream<java.lang.Long> to: java.util.stream.Stream<java.lang.Number>
 @ line 6, column 26.
         Stream<Number> x = Arrays.stream(new Integer[]{1}).flatMap((y) -> Arrays.stream(new Long[]{(long) 1}));
                            ^

1 error

Expected behavior

Compile successfully

The code should pass because the signature of the "flatMap" function is "<R> Stream<R> flatMap​(Function<? super T,​? extends Stream<? extends R>> mapper)". And therefore, Function<Integer, Stream<Long>> is a subtype of Function<? super Integer, ? extends Stream<? extends Number>.

Tested against master (commit: 5a5726342adeb37a6fbaa8cdcbe2d47dee8dc56c)