Generics cast issue

[tiller]

The code at https://github.com/Tiller/jdt-annot-issue/tree/genericscast gives the following error in eclipse (4.37 & 4.38 nightly, haven't tested before) but works with javac

Cannot cast from Enum<Enum> to Test.MyEnum Test.java /test/src/main/java/test line 16 Java Problem

    public void test() {
        assertThatEquals((MyEnum) enumExpression.valueOf("test.Test.MyEnum", "A")).test(MyEnum.A);
    }

    public static <T> Predicate<T> assertThatEquals(T elm) {
        return elm2 -> Objects.equals(elm, elm2);
    }

    public <T extends Enum<T>> T valueOf(String fqdn, String value) {
      ...
    }

Workaround is to not cast but explicitly provide the type:

assertThatEquals(enumExpression.<MyEnum>valueOf("test.Test.MyEnum", "A")).test(MyEnum.A);

(sorry for all the issues guys, I have had some time this week so I create what I have laying around, I have at most one other to create :D)

[srikanth-sankaran]

No problem. As long simple reproducers are attached that don't call for maven, gradle etc we welcome defect reports that are well written and showcasing real problems.

[srikanth-sankaran]

@tiller - Are you able to provide a standalone test case ? On the 3-4 issues I have extracted the core issue by stubbing out the various classes and methods. You can study the regression tests added to see what I did - this would speed up investigation. Our Generics and Type inference expert is a volunteer (@stephan-herrmann) who has tirelessly spent countless hours of his personal time to maintain generics support. Thanks for any help you are able to offer here - we are somewhat resource constrained.

[srikanth-sankaran]

Here is a reproducer extracted by co-pilot:

import java.util.Objects;
import java.util.function.Predicate;

public class X {
    
    public enum MyEnum {
        A, B, C;
    }
    
    static class EnumExpression {
        public <T extends Enum<T>> T valueOf(String fqdn, String value) {
            // Simplified implementation - just returns the enum constant
            return (T) MyEnum.valueOf(value);
        }
    }
    
    public static void main(String[] args) {
        X test = new X();
        test.test();
    }
    
    public void test() {
        EnumExpression enumExpression = new EnumExpression();
        
        // This should compile but Eclipse incorrectly reports:
        // "Cannot cast from Enum<Enum<T>> to X.MyEnum"
        assertThatEquals((MyEnum) enumExpression.valueOf("test.X.MyEnum", "A")).test(MyEnum.A);
        
        System.out.println("Test passed!");
    }
    
    public static <T> Predicate<T> assertThatEquals(T elm) {
        return elm2 -> {
            boolean result = Objects.equals(elm, elm2);
            System.out.println("Equals check: " + elm + " == " + elm2 + " -> " + result);
            return result;
        };
    }
}

Compiles with JDK25 with this message:

$ ~/jdk/jdk-25/jdk-25.0.1/bin/javac -Xlint:unchecked -g X.java
X.java:13: warning: [unchecked] unchecked cast
            return (T) MyEnum.valueOf(value);
                                     ^
  required: T
  found:    MyEnum
  where T is a type-variable:
    T extends Enum<T> declared in method <T>valueOf(String,String)
1 warning

Runtime output:

$ ~/jdk/jdk-25/jdk-25.0.1/bin/java X
Equals check: A == A -> true
Test passed!

With eclipse, we get a compile error:

Cannot cast from Enum<Enum<T>> to X.MyEnum

[stephan-herrmann]

Enum types recently came into focus, see last para of #5004 (comment):

At this point only two tests fail due to disabling the tweak:

• org.eclipse.jdt.core.tests.compiler.regression.GenericsRegressionTest.test434118()
• org.eclipse.jdt.core.tests.compiler.regression.GenericsRegressionTest_9.testIssue4864()

Both tests use enums in special ways. Is that an indication for the cause??

This makes all these tests good candidates for trying #4838