Description Éamonn McManus 2019-05-27 19:42:37 EDT

Created attachment 278758 [details]
Maven project reproducing the problem

The compiler is incorrectly reporting to annotation processors that an annotation on a type parameter also appears on uses of that type parameter. For example, consider this class:

package bug;

import com.google.auto.value.AutoValue;
import com.google.errorprone.annotations.ImmutableTypeParameter;

@AutoValue
abstract class Bug<@ImmutableTypeParameter T> {
  abstract T bug();

  static <@ImmutableTypeParameter T> Bug<T> of(T t) {
    return new AutoValue_Bug<>(t);
  }
}

@ImmutableTypeParameter has @Target({TYPE_PARAMETER}), so it is only applicable to type parameters. But when an annotation processor looks at the T that is the return type of the bug() method, it gets back a TypeVariable with the @ImmutableTypeParameter annotation. This leads AutoValue to generate this code:

package bug;

import com.google.errorprone.annotations.ImmutableTypeParameter;
import javax.annotation.Generated;

@Generated("com.google.auto.value.processor.AutoValueProcessor")
final class AutoValue_Bug<@ImmutableTypeParameter T> extends Bug<T> {

  private final @ImmutableTypeParameter T bug;

  AutoValue_Bug(
      @ImmutableTypeParameter T bug) {
    if (bug == null) {
      throw new NullPointerException("Null bug");
    }
    this.bug = bug;
  }

  @Override
  @ImmutableTypeParameter T bug() {
    return bug;
  }
  ...
}

The <@ImmutableTypeParameter T> in the class declaration is correctly copied, because it was returned from TypeParameterElement.getAnnotationMirrors(). However, the other three occurrences of @ImmutableTypeParameter are incorrect, and come about because TypeVariable.getAnnotationMirrors() reported that the type variable T has that annotation. This code then fails to compile, because the compiler correctly rejects @ImmutableTypeParameter applied to a type variable. It can only be applied to a type parameter.

This code compiles fine with javac.