javac does not initialize receiver types of methods read from class files, which results in ExecutableElement.getReceiverType incorrectly reporting that methods don't have a receiver type.

Repro:

=== P.java
import java.util.Set;
import javax.annotation.processing.AbstractProcessor;
import javax.annotation.processing.RoundEnvironment;
import javax.annotation.processing.SupportedAnnotationTypes;
import javax.lang.model.SourceVersion;
import javax.lang.model.element.Element;
import javax.lang.model.element.ExecutableElement;
import javax.lang.model.element.TypeElement;
import javax.tools.Diagnostic;

@SupportedAnnotationTypes("*")
public class P extends AbstractProcessor {
  @Override
  public SourceVersion getSupportedSourceVersion() {
    return SourceVersion.latestSupported();
  }

  boolean first = true;

  @Override
  public boolean process(Set<? extends TypeElement> annotations, RoundEnvironment roundEnv) {
    if (first) {
      for (Element e :
          processingEnv.getElementUtils().getTypeElement("Test").getEnclosedElements()) {
        if (e.getSimpleName().contentEquals("f")) {
          ExecutableElement f = (ExecutableElement) e;
          processingEnv
              .getMessager()
              .printMessage(Diagnostic.Kind.NOTE, f.getReceiverType().toString(), f);
        }
      }
      first = false;
    }
    return false;
  }
}
=== Test.java
class Test {
  void f(Test this) {}
}
===

The receiver type is available if 'Test' is compiled from source:

$ javac -processor P -implicit:none Test.java
Test.java:2: Note: Test
  void f(Test this) {}
       ^

The receiver type is incorrectly reported as 'none' if 'Test' is loaded from the classpath:

$ javac -processor P -implicit:none Test
Note: none