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Closure Compiler is a JavaScript optimizing compiler. It parses your JavaScript, analyzes it, removes dead code and rewrites and minimizes what's left. It also checks syntax, variable references, and types, and warns about common JavaScript pitfalls. It is used in many of Google's JavaScript apps, including Gmail, Google Web Search, Google Maps, and Google Docs.

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/*
 *
 * ***** BEGIN LICENSE BLOCK *****
 * Version: MPL 1.1/GPL 2.0
 *
 * The contents of this file are subject to the Mozilla Public License Version
 * 1.1 (the "License"); you may not use this file except in compliance with
 * the License. You may obtain a copy of the License at
 * http://www.mozilla.org/MPL/
 *
 * Software distributed under the License is distributed on an "AS IS" basis,
 * WITHOUT WARRANTY OF ANY KIND, either express or implied. See the License
 * for the specific language governing rights and limitations under the
 * License.
 *
 * The Original Code is Rhino code, released
 * May 6, 1999.
 *
 * The Initial Developer of the Original Code is
 * Netscape Communications Corporation.
 * Portions created by the Initial Developer are Copyright (C) 1997-1999
 * the Initial Developer. All Rights Reserved.
 *
 * Contributor(s):
 *   Bob Jervis
 *   Google Inc.
 *
 * Alternatively, the contents of this file may be used under the terms of
 * the GNU General Public License Version 2 or later (the "GPL"), in which
 * case the provisions of the GPL are applicable instead of those above. If
 * you wish to allow use of your version of this file only under the terms of
 * the GPL and not to allow others to use your version of this file under the
 * MPL, indicate your decision by deleting the provisions above and replacing
 * them with the notice and other provisions required by the GPL. If you do
 * not delete the provisions above, a recipient may use your version of this
 * file under either the MPL or the GPL.
 *
 * ***** END LICENSE BLOCK ***** */
package com.google.javascript.rhino.jstype;

import static com.google.javascript.jscomp.base.JSCompObjects.identical;

import com.google.common.collect.Sets;
import com.google.javascript.rhino.jstype.ContainsUpperBoundSuperTypeVisitor.Result;
import java.util.Set;
import org.jspecify.nullness.Nullable;

/**
 * A type visitor that traverse through the referenced types of "forwaring types" to search for a
 * target type.
 *
 * The result of the visitation indicates whether the target type was present, absent, or
 * undcidable due to a cycle.
 *
 * This is used by {@link TemplateType} as a helper in cycle detection and subtyping. Cycles are
 * checked for once after TemplateTypes are constructed in FunctionTypeBuilder and an error is
 * emitted if they are found.
 */
final class ContainsUpperBoundSuperTypeVisitor extends Visitor.WithDefaultCase {

  private final JSType target;
  private final Set seen = Sets.newIdentityHashSet();

  public ContainsUpperBoundSuperTypeVisitor(JSType target) {
    this.target = target;
  }

  @Override
  protected Result caseDefault(@Nullable JSType type) {
    if (this.target == null) {
      return Result.ABSENT;
    }

    return identical(type, this.target) ? Result.PRESENT : Result.ABSENT;
  }

  @Override
  public Result caseTemplateType(TemplateType type) {
    return caseForwardingType(type, type.getBound());
  }

  @Override
  public Result caseNamedType(NamedType type) {
    return caseForwardingType(type, type.getReferencedType());
  }

  @Override
  public Result caseTemplatizedType(TemplatizedType type) {
    return caseForwardingType(type, type.getReferencedType());
  }

  @Override
  public Result caseUnionType(UnionType type) {
    if (identical(type, target)) {
      return Result.PRESENT;
    }

    for (JSType alt : type.getAlternates()) {
      Result foundInAlt = alt.visit(this);
      if (foundInAlt != Result.ABSENT) {
        return foundInAlt;
      }
    }

    return Result.ABSENT;
  }

  private Result caseForwardingType(JSType type, JSType reference) {
    if (identical(type, target)) {
      return Result.PRESENT;
    } else if (seen.contains(type)) {
      return Result.CYCLE;
    } else {
      seen.add(type);
      return reference.visit(this);
    }
  }

  /** Represents the outcome of a visitation of the {@link ContainsUpperBoundSuperTypeVisitor}. */
  enum Result {
    PRESENT,
    ABSENT,
    /** Containment is undecidable due to a reference cycle. */
    CYCLE;
  }
}