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The Checker Framework enhances Java's type system to make it more powerful and useful. This lets software developers detect and prevent errors in their Java programs. The Checker Framework includes compiler plug-ins ("checkers") that find bugs or verify their absence. It also permits you to write your own compiler plug-ins.

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package org.checkerframework.framework.qual;

import java.lang.annotation.Documented;
import java.lang.annotation.ElementType;
import java.lang.annotation.Retention;
import java.lang.annotation.RetentionPolicy;
import java.lang.annotation.Target;

/**
 * NOTE: This meta-annotation is not currently
 * enforced.
 *
 * A meta-annotation that restricts the type-use locations where a type qualifier may be written.
 * When written together with {@code @Target({ElementType.TYPE_USE})}, the given type qualifier may
 * be written only at locations listed in the {@code @TargetLocations(...)} meta-annotation.
 * {@code @Target({ElementType.TYPE_USE})} together with no {@code @TargetLocations(...)} means that
 * the qualifier can be written on any type use.
 *
 * 
This enables a type system designer to permit a qualifier to be written only in certain
 * locations. For example, some type systems' top and bottom qualifier (such as {@link
 * org.checkerframework.checker.nullness.qual.KeyForBottom}) should only be written on an explicit
 * wildcard upper or lower bound. This meta-annotation is a declarative, coarse-grained approach to
 * enable that. For finer-grained control, override {@code visit*} methods that visit trees in
 * BaseTypeVisitor.
 *
 * This annotation does not prevent the type system from defaulting, inferring, or computing the
 * given type annotation at the given location. It only prevents users from writing an explicit
 * annotation at the given location.
 */
@Documented
@Retention(RetentionPolicy.RUNTIME)
@Target(ElementType.ANNOTATION_TYPE)
public @interface TargetLocations {
  /** Type uses at which the qualifier is permitted to be written in source code. */
  TypeUseLocation[] value();
}