Version

Eclipse Compiler 3.33.0
(Maven: org.eclipse.jdt:ecj:3.33.0)

Description

Preconditions:

• a generic method M of a local class ...
• contains a lambda expression ...
• which creates an anonymous class C of a generic class ...
• which captures a type variable from M

In that case reflection erroneously returns null as type argument for C.
If any of the above is changed, e.g. not using a lambda, or not using a local class, then it looks like this cannot be reproduced.

Have reported this for the Eclipse compiler because the same code seems to work as expected when compiling and running using JDK 17 (though same error occurs with JDK 11, see JDK-8306438). But maybe this is actually a bug in the Java reflection library.

Reproduction steps

1. Create a file called Test.java with the following content

   import java.lang.reflect.*;
   
   class Test {
       static class Capturing<T> {
           protected Capturing() {
               ParameterizedType paramT = (ParameterizedType) getClass().getGenericSuperclass();
               Type t = paramT.getActualTypeArguments()[0];
   
               if (t instanceof TypeVariable) {
                   System.out.println("Found expected type");
               } else {
                   throw new AssertionError("Unexpected type: " + t);
               }
           }
       }
   
       static void run(Runnable r) {
           r.run();
       }
   
       public static <T> void main(String... args) {
           class Local {
               <M> void runTest() {
                   run(() -> new Capturing<M>() {});
               }
           }
   
           new Local().runTest();
       }
   }

2. Compile the class with the Eclipse compiler

   java -jar ./ecj-3.33.0.jar -17 ./Test.java
   

3. Try to run the compiled class using Java 17

   java Test

   ❌ Bug: It fails with AssertionError: Unexpected type: null