When producing a LinkageError due to a ClassLoader constraint violation during method resolution, I get the following error message:

Exception in thread "main" java.lang.LinkageError: loader constraint violation: when resolving method "B.m()LFoo;" the class loader (instance of PreemptingClassLoader) of the current class, Task, and the class loader (instance of sun/misc/Launcher$AppClassLoader) for resolved class, B, have different Class objects for the type sun/misc/Launcher$AppClassLoader used in the signature
at Task.run(Task.java:8)
at Test.main(Test.java:7)

Two problems:
1) sun/misc/Launcher$AppClassLoader is _not_ the ClassLoader of B. It is the ClassLoader of the resolved method's class, A (which is a supertype of B).

2) "the type sun/misc/Launcher$AppClassLoader used in the signature" should read "the type Foo used in the signature"

Sources to reproduce follow.

----------

public class Foo {}

----------

public class A {
  public Foo m() { return null; }
}

----------

public class B extends A {}

----------

public class Task implements Runnable { public void run() {

Class<?> c = Foo.class;
B b = new B(); b.m();

}}

----------

public class Test { public static void main(String... args) throws Exception {

Class<?> c = Foo.class;

ClassLoader l = new PreemptingClassLoader("B", "Foo", "Task");
Runnable r = (Runnable) l.loadClass("Task").newInstance();
r.run();

}}

----------

import java.util.*;
import java.io.*;

public class PreemptingClassLoader extends ClassLoader {

  private final Set<String> names = new HashSet<>();
  
  public PreemptingClassLoader(String... names) {
    for (String n : names) this.names.add(n);
  }
  
  protected Class<?> loadClass(String name, boolean resolve) throws ClassNotFoundException {
    if (!names.contains(name)) return super.loadClass(name, resolve);
    Class<?> result = findLoadedClass(name);
    if (result == null) {
      String filename = name.replace('.', '/') + ".class";
      try (InputStream data = getResourceAsStream(filename)) {
        if (data == null) throw new ClassNotFoundException();
        try (ByteArrayOutputStream buffer = new ByteArrayOutputStream()) {
          int b;
          do {
            b = data.read();
            if (b >= 0) buffer.write(b);
          } while (b >= 0);
          byte[] bytes = buffer.toByteArray();
          result = defineClass(name, bytes, 0, bytes.length);
        }
      }
      catch (IOException e) {
        throw new ClassNotFoundException("Error reading class file", e);
      }
    }
    if (resolve) resolveClass(result);
    return result;
  }

}

----------