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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;

/**
 * A marker annotation, written on a class declaration, that signifies that one or more of the
 * class's type parameters can be treated covariantly. For example, if {@code MyClass} has a single
 * type parameter that is treated covariantly, and if {@code B} is a subtype of {@code A}, then
 * {@code SomeClass} is a subtype of {@code SomeClass}.
 *
 * Ordinarily, Java treats type parameters invariantly: {@code SomeClass} is unrelated to
 * (neither a subtype nor a supertype of) {@code SomeClass}.
 *
 * 
It is only safe to mark a type parameter as covariant if the type parameter is used in a
 * read-only way: values of that type are read from but never modified. This property is not
 * checked; the {@code @Covariant} is simply trusted.
 *
 * 
Here is an example use:
 *
 * 
{@code @Covariant(0)
 * public interface Iterator { ... }
 * }

 *
 * @checker_framework.manual #covariant-type-parameters Covariant type parameters
 */
@Documented
@Target({ElementType.TYPE})
@Retention(RetentionPolicy.RUNTIME)
public @interface Covariant {
    /** The zero-based indices of the type parameters that should be treated covariantly. */
    int[] value();
}