ADDITIONAL SYSTEM INFORMATION :
Linux 5.4.74-1-MANJARO #1 SMP PREEMPT Sun Nov 1 13:43:13 UTC 2020 x86_64 GNU/Linux
OpenJDK 8-15, Oracle JDK 11.0.9, Oracle JDK 15.0.1
Default configuration
Dependencies: java.lang.reflect.ParameterizedType, java.lang.reflect.Type


A DESCRIPTION OF THE PROBLEM :
Description of the bug can be found at https://github.com/albsch/java_types_matrix

For Java versions <13 getActualTypeArguments returns an array filled with null values.
This causes different java versions to have different semantics for the same source file (see attached code).


STEPS TO FOLLOW TO REPRODUCE THE PROBLEM :
Put the source code of the two classes into the test folder

Compile classes with Java 8-12 (javac test/*.java)
Run with Java 8-12, throws Exception (java test.Main)
Compile classes with Java 13-15 (javac test/*.java)
Run with Java 13-15, does not throw Exception (java test.Main)

EXPECTED VERSUS ACTUAL BEHAVIOR :
EXPECTED -
Same result for all supported Java version (either no exception thrown or exception thrown)
ACTUAL -
Different evaluation of java source code depending on java version
Java 8-13: exception thrown (actualTypeArguments array is filled with null values)
Java 13-15: No exception thrown (actualTypeArguments array is not filled with null values)

---------- BEGIN SOURCE ----------
Main class:
---

package test;
import test.T;
public class Main {
  public static void main(String[] args) { run(); }

  public static <A> void run() {
    Runnable r = () -> {
        T<A> t = new T<A>(){};
        if(t.type == null) throw new IllegalStateException();
    };
    r.run();
  }
}



Token class:
---
package test;
import java.lang.reflect.ParameterizedType;
import java.lang.reflect.Type;

public abstract class T<X> {

    public final Type type;

    public T() {
        this.type = resolveType();
    }

    // this resolves and saves the type with generics at runtime
    public Type resolveType() {
        final Type genericSuperclass = getClass().getGenericSuperclass();
        final ParameterizedType parameterizedGenericSuperclass = (ParameterizedType) genericSuperclass;
        final Type[] actualTypeArguments = parameterizedGenericSuperclass.getActualTypeArguments();
        return actualTypeArguments[0];
    }
}
---------- END SOURCE ----------

CUSTOMER SUBMITTED WORKAROUND :
No workaround found.

FREQUENCY : always