After JDK-8206325, annotations whose symbols can't be resolved during the current processing round are not returned by getAnnotationMirrors(). I noticed this investigating an annotation processor that stopped working with the new behaviour, because it assumes a generated annotation will be returned by getAnnotationMirrors on the first processing round, before the annotation is actually generated.

I think it's somewhat subjective whether this is a bug, there are cases where the processing model can't return information when errors are present, but in this case I wonder if it would be better to include the erroneous annotation in the result of getAnnotationMirrors, and let clients check if the annotation is erroneous?

A possible fix is to modify the check in Annotation.annotateNow to do something like:

-if (a.type.tsym.isAnnotationType()) {
+if (a.type.isErroneous() || a.type.tsym.isAnnotationType()) {

Repro:

===
import java.io.IOException;
import java.io.UncheckedIOException;
import java.io.Writer;
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.lang.model.element.VariableElement;
import javax.tools.Diagnostic.Kind;
import javax.tools.JavaFileObject;

@SupportedAnnotationTypes("*")
public class P extends AbstractProcessor {

  @Override
  public SourceVersion getSupportedSourceVersion() {
    return SourceVersion.latestSupported();
  }

  int round = 1;

  @Override
  public boolean process(Set<? extends TypeElement> annotations, RoundEnvironment roundEnv) {
    processingEnv.getMessager().printMessage(Kind.NOTE, "round " + round);
    Element t = processingEnv.getElementUtils().getTypeElement("T");
    for (Element e : t.getEnclosedElements()) {
      if (e instanceof ExecutableElement) {
        for (VariableElement p : ((ExecutableElement) e).getParameters()) {
          processingEnv
              .getMessager()
              .printMessage(
                  Kind.NOTE, "annotations on " + p + ": " + p.getAnnotationMirrors().toString());
        }
      }
    }
    if (round == 1) {
      String name = "A";
      try {
        JavaFileObject jfo = processingEnv.getFiler().createSourceFile(name);
        try (Writer w = jfo.openWriter()) {
          w.write("@interface " + name + " {}");
        }
      } catch (IOException e) {
        throw new UncheckedIOException(e);
      }
    }
    round++;
    return false;
  }
}
===

===
class T {
  void f(@A int x) {}
}
===

With javac before JDK-8206325, the annotation @A on the parameter x is visible during the first processing round, before it has been generated:

$ javac -fullversion -processor P T.java -implicit:none -s gen
javac full version "12-ea+22"
Note: round 1
Note: annotations on x: @A
Note: round 2
Note: annotations on x: @A
Note: round 3
Note: annotations on x: @A

After the fix, the annotation is dropped on the first round. A processor that visits x on the first round has no way of telling that x has a (currently erroneous) annotation, and might assume x is well formed but has no annotations:

$ javac -fullversion -processor P T.java -implicit:none -s gen
javac full version "12-ea+23"
Note: round 1
Note: annotations on x:
Note: round 2
Note: annotations on x: @A
Note: round 3
Note: annotations on x: @A