path: root/common/src/main/java/com/google/auto/common/MoreTypes.java

/*
 * Copyright 2014 Google LLC
 *
 * Licensed under the Apache License, Version 2.0 (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.apache.org/licenses/LICENSE-2.0
 *
 * Unless required by applicable law or agreed to in writing, software
 * distributed under the License is distributed on an "AS IS" BASIS,
 * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
 * See the License for the specific language governing permissions and
 * limitations under the License.
 */
package com.google.auto.common;

import static com.google.common.base.Preconditions.checkNotNull;
import static com.google.common.base.Preconditions.checkState;
import static javax.lang.model.type.TypeKind.ARRAY;
import static javax.lang.model.type.TypeKind.DECLARED;
import static javax.lang.model.type.TypeKind.EXECUTABLE;
import static javax.lang.model.type.TypeKind.INTERSECTION;
import static javax.lang.model.type.TypeKind.TYPEVAR;
import static javax.lang.model.type.TypeKind.WILDCARD;

import com.google.common.base.Equivalence;
import com.google.common.base.Optional;
import com.google.common.collect.ImmutableList;
import com.google.common.collect.ImmutableSet;
import java.util.HashSet;
import java.util.Iterator;
import java.util.List;
import java.util.Set;
import javax.lang.model.element.Element;
import javax.lang.model.element.ElementKind;
import javax.lang.model.element.ExecutableElement;
import javax.lang.model.element.Modifier;
import javax.lang.model.element.TypeElement;
import javax.lang.model.element.TypeParameterElement;
import javax.lang.model.element.VariableElement;
import javax.lang.model.type.ArrayType;
import javax.lang.model.type.DeclaredType;
import javax.lang.model.type.ErrorType;
import javax.lang.model.type.ExecutableType;
import javax.lang.model.type.IntersectionType;
import javax.lang.model.type.NoType;
import javax.lang.model.type.NullType;
import javax.lang.model.type.PrimitiveType;
import javax.lang.model.type.TypeKind;
import javax.lang.model.type.TypeMirror;
import javax.lang.model.type.TypeVariable;
import javax.lang.model.type.WildcardType;
import javax.lang.model.util.Elements;
import javax.lang.model.util.SimpleTypeVisitor8;
import javax.lang.model.util.Types;
import org.checkerframework.checker.nullness.qual.Nullable;

/**
 * Utilities related to {@link TypeMirror} instances.
 *
 * @author Gregory Kick
 * @since 2.0
 */
public final class MoreTypes {
  private static final class TypeEquivalence extends Equivalence<TypeMirror> {
    private static final TypeEquivalence INSTANCE = new TypeEquivalence();

    @Override
    protected boolean doEquivalent(TypeMirror a, TypeMirror b) {
      return MoreTypes.equal(a, b, ImmutableSet.<ComparedElements>of());
    }

    @Override
    protected int doHash(TypeMirror t) {
      return MoreTypes.hash(t, ImmutableSet.<Element>of());
    }

    @Override
    public String toString() {
      return "MoreTypes.equivalence()";
    }
  }

  /**
   * Returns an {@link Equivalence} that can be used to compare types. The standard way to compare
   * types is {@link javax.lang.model.util.Types#isSameType Types.isSameType}, but this alternative
   * may be preferred in a number of cases:
   *
   * <ul>
   * <li>If you don't have an instance of {@code Types}.
   * <li>If you want a reliable {@code hashCode()} for the types, for example to construct a set
   *     of types using {@link java.util.HashSet} with {@link Equivalence#wrap(Object)}.
   * <li>If you want distinct type variables to be considered equal if they have the same names
   *     and bounds.
   * <li>If you want wildcard types to compare equal if they have the same bounds. {@code
   *     Types.isSameType} never considers wildcards equal, even when comparing a type to itself.
   * </ul>
   */
  public static Equivalence<TypeMirror> equivalence() {
    return TypeEquivalence.INSTANCE;
  }

  // So EQUAL_VISITOR can be a singleton, we maintain visiting state, in particular which types
  // have been seen already, in this object.
  // The logic for handling recursive types like Comparable<T extends Comparable<T>> is very tricky.
  // If we're not careful we'll end up with an infinite recursion. So we record the types that
  // we've already seen during the recursion, and if we see the same pair of types again we just
  // return true provisionally. But "the same pair of types" is itself poorly-defined. We can't
  // just say that it is an equal pair of TypeMirrors, because of course if we knew how to
  // determine that then we wouldn't need the complicated type visitor at all. On the other hand,
  // we can't say that it is an identical pair of TypeMirrors either, because there's no
  // guarantee that the TypeMirrors for the two Ts in Comparable<T extends Comparable<T>> will be
  // represented by the same object, and indeed with the Eclipse compiler they aren't. We could
  // compare the corresponding Elements, since equality is well-defined there, but that's not enough
  // either, because the Element for Set<Object> is the same as the one for Set<String>. So we
  // approximate by comparing the Elements and, if there are any type arguments, requiring them to
  // be identical. This may not be foolproof either but it is sufficient for all the cases we've
  // encountered so far.
  private static final class EqualVisitorParam {
    TypeMirror type;
    Set<ComparedElements> visiting;
  }

  private static class ComparedElements {
    final Element a;
    final ImmutableList<TypeMirror> aArguments;
    final Element b;
    final ImmutableList<TypeMirror> bArguments;

    ComparedElements(
        Element a,
        ImmutableList<TypeMirror> aArguments,
        Element b,
        ImmutableList<TypeMirror> bArguments) {
      this.a = a;
      this.aArguments = aArguments;
      this.b = b;
      this.bArguments = bArguments;
    }

    @Override
    public boolean equals(@Nullable Object o) {
      if (!(o instanceof ComparedElements)) {
        return false;
      }
      ComparedElements that = (ComparedElements) o;

      int nArguments = this.aArguments.size();
      if (nArguments != that.aArguments.size()) {
        return false;
      }
      // The arguments must be the same size, but we check anyway.
      if (nArguments != this.bArguments.size() || nArguments != that.bArguments.size()) {
        return false;
      }

      if (!this.a.equals(that.a) || !this.b.equals(that.b)) {
        return false;
      }

      /*
       * The purpose here is just to avoid the infinite recursion that we would otherwise have
       * if Enum<E extends Enum<E>> is compared against itself, for example. If we are able to
       * see that the inner Enum<E> is the same object as the outer one then we don't need a
       * recursive call to compare the "a" Enum<E> against the "b" Enum<E>. The same-object check
       * may not be completely justified, but it relies on the practical assumption that the
       * compiler is not going to conjure up an infinite regress of objects to represent this
       * recursive type. Other comparison methods like comparing their toString() are expensive
       * and not warranted.
       */
      for (int i = 0; i < nArguments; i++) {
        if (this.aArguments.get(i) != that.aArguments.get(i)) {
          return false;
        }
        if (this.bArguments.get(i) != that.bArguments.get(i)) {
          return false;
        }
      }

      return true;
    }

    @Override
    public int hashCode() {
      return a.hashCode() * 31 + b.hashCode();
    }
  }

  private static final class EqualVisitor extends SimpleTypeVisitor8<Boolean, EqualVisitorParam> {
    private static final EqualVisitor INSTANCE = new EqualVisitor();

    @Override
    protected Boolean defaultAction(TypeMirror a, EqualVisitorParam p) {
      return a.getKind().equals(p.type.getKind());
    }

    @Override
    public Boolean visitArray(ArrayType a, EqualVisitorParam p) {
      if (p.type.getKind().equals(ARRAY)) {
        ArrayType b = (ArrayType) p.type;
        return equal(a.getComponentType(), b.getComponentType(), p.visiting);
      }
      return false;
    }

    @Override
    public Boolean visitDeclared(DeclaredType a, EqualVisitorParam p) {
      if (p.type.getKind().equals(DECLARED)) {
        DeclaredType b = (DeclaredType) p.type;
        Element aElement = a.asElement();
        Element bElement = b.asElement();
        Set<ComparedElements> newVisiting =
            visitingSetPlus(
                p.visiting, aElement, a.getTypeArguments(), bElement, b.getTypeArguments());
        if (newVisiting.equals(p.visiting)) {
          // We're already visiting this pair of elements.
          // This can happen for example with Enum in Enum<E extends Enum<E>>. Return a
          // provisional true value since if the Elements are not in fact equal the original
          // visitor of Enum will discover that. We have to check both Elements being compared
          // though to avoid missing the fact that one of the types being compared
          // differs at exactly this point.
          return true;
        }
        return aElement.equals(bElement)
            && equal(enclosingType(a), enclosingType(b), newVisiting)
            && equalLists(a.getTypeArguments(), b.getTypeArguments(), newVisiting);
      }
      return false;
    }

    @Override
    @SuppressWarnings("TypeEquals")
    public Boolean visitError(ErrorType a, EqualVisitorParam p) {
      return a.equals(p.type);
    }

    @Override
    public Boolean visitExecutable(ExecutableType a, EqualVisitorParam p) {
      if (p.type.getKind().equals(EXECUTABLE)) {
        ExecutableType b = (ExecutableType) p.type;
        return equalLists(a.getParameterTypes(), b.getParameterTypes(), p.visiting)
            && equal(a.getReturnType(), b.getReturnType(), p.visiting)
            && equalLists(a.getThrownTypes(), b.getThrownTypes(), p.visiting)
            && equalLists(a.getTypeVariables(), b.getTypeVariables(), p.visiting);
      }
      return false;
    }

    @Override
    public Boolean visitIntersection(IntersectionType a, EqualVisitorParam p) {
      if (p.type.getKind().equals(INTERSECTION)) {
        IntersectionType b = (IntersectionType) p.type;
        return equalLists(a.getBounds(), b.getBounds(), p.visiting);
      }
      return false;
    }

    @Override
    public Boolean visitTypeVariable(TypeVariable a, EqualVisitorParam p) {
      if (p.type.getKind().equals(TYPEVAR)) {
        TypeVariable b = (TypeVariable) p.type;
        TypeParameterElement aElement = (TypeParameterElement) a.asElement();
        TypeParameterElement bElement = (TypeParameterElement) b.asElement();
        Set<ComparedElements> newVisiting = visitingSetPlus(p.visiting, aElement, bElement);
        if (newVisiting.equals(p.visiting)) {
          // We're already visiting this pair of elements.
          // This can happen with our friend Eclipse when looking at <T extends Comparable<T>>.
          // It incorrectly reports the upper bound of T as T itself.
          return true;
        }
        // We use aElement.getBounds() instead of a.getUpperBound() to avoid having to deal with
        // the different way intersection types (like <T extends Number & Comparable<T>>) are
        // represented before and after Java 8. We do have an issue that this code may consider
        // that <T extends Foo & Bar> is different from <T extends Bar & Foo>, but it's very
        // hard to avoid that, and not likely to be much of a problem in practice.
        return equalLists(aElement.getBounds(), bElement.getBounds(), newVisiting)
            && equal(a.getLowerBound(), b.getLowerBound(), newVisiting)
            && a.asElement().getSimpleName().equals(b.asElement().getSimpleName());
      }
      return false;
    }

    @Override
    public Boolean visitWildcard(WildcardType a, EqualVisitorParam p) {
      if (p.type.getKind().equals(WILDCARD)) {
        WildcardType b = (WildcardType) p.type;
        return equal(a.getExtendsBound(), b.getExtendsBound(), p.visiting)
            && equal(a.getSuperBound(), b.getSuperBound(), p.visiting);
      }
      return false;
    }

    @Override
    public Boolean visitUnknown(TypeMirror a, EqualVisitorParam p) {
      throw new UnsupportedOperationException();
    }

    private Set<ComparedElements> visitingSetPlus(
        Set<ComparedElements> visiting, Element a, Element b) {
      ImmutableList<TypeMirror> noArguments = ImmutableList.of();
      return visitingSetPlus(visiting, a, noArguments, b, noArguments);
    }

    private Set<ComparedElements> visitingSetPlus(
        Set<ComparedElements> visiting,
        Element a,
        List<? extends TypeMirror> aArguments,
        Element b,
        List<? extends TypeMirror> bArguments) {
      ComparedElements comparedElements =
          new ComparedElements(
              a, ImmutableList.<TypeMirror>copyOf(aArguments),
              b, ImmutableList.<TypeMirror>copyOf(bArguments));
      Set<ComparedElements> newVisiting = new HashSet<ComparedElements>(visiting);
      newVisiting.add(comparedElements);
      return newVisiting;
    }
  }

  @SuppressWarnings("TypeEquals")
  private static boolean equal(
      @Nullable TypeMirror a, @Nullable TypeMirror b, Set<ComparedElements> visiting) {
    if (a == b) {
      return true;
    }
    if (a == null || b == null) {
      return false;
    }
    // TypeMirror.equals is not guaranteed to return true for types that are equal, but we can
    // assume that if it does return true then the types are equal. This check also avoids getting
    // stuck in infinite recursion when Eclipse decrees that the upper bound of the second K in
    // <K extends Comparable<K>> is a distinct but equal K.
    // The javac implementation of ExecutableType, at least in some versions, does not take thrown
    // exceptions into account in its equals implementation, so avoid this optimization for
    // ExecutableType.
    @SuppressWarnings("TypesEquals")
    boolean equal = a.equals(b);
    if (equal && a.getKind() != TypeKind.EXECUTABLE) {
      return true;
    }
    EqualVisitorParam p = new EqualVisitorParam();
    p.type = b;
    p.visiting = visiting;
    return a.accept(EqualVisitor.INSTANCE, p);
  }

  /**
   * Returns the type of the innermost enclosing instance, or null if there is none. This is the
   * same as {@link DeclaredType#getEnclosingType()} except that it returns null rather than
   * NoType for a static type. We need this because of
   * <a href="https://bugs.eclipse.org/bugs/show_bug.cgi?id=508222">this bug</a> whereby
   * the Eclipse compiler returns a value for static classes that is not NoType.
   */
  private static @Nullable TypeMirror enclosingType(DeclaredType t) {
    TypeMirror enclosing = t.getEnclosingType();
    if (enclosing.getKind().equals(TypeKind.NONE)
        || t.asElement().getModifiers().contains(Modifier.STATIC)) {
      return null;
    }
    return enclosing;
  }

  private static boolean equalLists(
      List<? extends TypeMirror> a, List<? extends TypeMirror> b, Set<ComparedElements> visiting) {
    int size = a.size();
    if (size != b.size()) {
      return false;
    }
    // Use iterators in case the Lists aren't RandomAccess
    Iterator<? extends TypeMirror> aIterator = a.iterator();
    Iterator<? extends TypeMirror> bIterator = b.iterator();
    while (aIterator.hasNext()) {
      // We checked that the lists have the same size, so we know that bIterator.hasNext() too.
      TypeMirror nextMirrorA = aIterator.next();
      TypeMirror nextMirrorB = bIterator.next();
      if (!equal(nextMirrorA, nextMirrorB, visiting)) {
        return false;
      }
    }
    return true;
  }

  private static final int HASH_SEED = 17;
  private static final int HASH_MULTIPLIER = 31;

  private static final class HashVisitor extends SimpleTypeVisitor8<Integer, Set<Element>> {
    private static final HashVisitor INSTANCE = new HashVisitor();

    int hashKind(int seed, TypeMirror t) {
      int result = seed * HASH_MULTIPLIER;
      result += t.getKind().hashCode();
      return result;
    }

    @Override
    protected Integer defaultAction(TypeMirror e, Set<Element> visiting) {
      return hashKind(HASH_SEED, e);
    }

    @Override
    public Integer visitArray(ArrayType t, Set<Element> visiting) {
      int result = hashKind(HASH_SEED, t);
      result *= HASH_MULTIPLIER;
      result += t.getComponentType().accept(this, visiting);
      return result;
    }

    @Override
    public Integer visitDeclared(DeclaredType t, Set<Element> visiting) {
      Element element = t.asElement();
      if (visiting.contains(element)) {
        return 0;
      }
      Set<Element> newVisiting = new HashSet<Element>(visiting);
      newVisiting.add(element);
      int result = hashKind(HASH_SEED, t);
      result *= HASH_MULTIPLIER;
      result += t.asElement().hashCode();
      result *= HASH_MULTIPLIER;
      result += t.getEnclosingType().accept(this, newVisiting);
      result *= HASH_MULTIPLIER;
      result += hashList(t.getTypeArguments(), newVisiting);
      return result;
    }

    @Override
    public Integer visitExecutable(ExecutableType t, Set<Element> visiting) {
      int result = hashKind(HASH_SEED, t);
      result *= HASH_MULTIPLIER;
      result += hashList(t.getParameterTypes(), visiting);
      result *= HASH_MULTIPLIER;
      result += t.getReturnType().accept(this, visiting);
      result *= HASH_MULTIPLIER;
      result += hashList(t.getThrownTypes(), visiting);
      result *= HASH_MULTIPLIER;
      result += hashList(t.getTypeVariables(), visiting);
      return result;
    }

    @Override
    public Integer visitTypeVariable(TypeVariable t, Set<Element> visiting) {
      int result = hashKind(HASH_SEED, t);
      result *= HASH_MULTIPLIER;
      result += t.getLowerBound().accept(this, visiting);
      TypeParameterElement element = (TypeParameterElement) t.asElement();
      for (TypeMirror bound : element.getBounds()) {
        result *= HASH_MULTIPLIER;
        result += bound.accept(this, visiting);
      }
      return result;
    }

    @Override
    public Integer visitWildcard(WildcardType t, Set<Element> visiting) {
      int result = hashKind(HASH_SEED, t);
      result *= HASH_MULTIPLIER;
      result += (t.getExtendsBound() == null) ? 0 : t.getExtendsBound().accept(this, visiting);
      result *= HASH_MULTIPLIER;
      result += (t.getSuperBound() == null) ? 0 : t.getSuperBound().accept(this, visiting);
      return result;
    }

    @Override
    public Integer visitUnknown(TypeMirror t, Set<Element> visiting) {
      throw new UnsupportedOperationException();
    }
  }

  private static int hashList(List<? extends TypeMirror> mirrors, Set<Element> visiting) {
    int result = HASH_SEED;
    for (TypeMirror mirror : mirrors) {
      result *= HASH_MULTIPLIER;
      result += hash(mirror, visiting);
    }
    return result;
  }

  private static int hash(TypeMirror mirror, Set<Element> visiting) {
    return mirror == null ? 0 : mirror.accept(HashVisitor.INSTANCE, visiting);
  }

  /**
   * Returns the set of {@linkplain TypeElement types} that are referenced by the given {@link
   * TypeMirror}.
   */
  public static ImmutableSet<TypeElement> referencedTypes(TypeMirror type) {
    checkNotNull(type);
    ImmutableSet.Builder<TypeElement> elements = ImmutableSet.builder();
    type.accept(ReferencedTypes.INSTANCE, elements);
    return elements.build();
  }

  private static final class ReferencedTypes
      extends SimpleTypeVisitor8<@Nullable Void, ImmutableSet.Builder<TypeElement>> {
    private static final ReferencedTypes INSTANCE = new ReferencedTypes();

    @Override
    public @Nullable Void visitArray(ArrayType t, ImmutableSet.Builder<TypeElement> p) {
      t.getComponentType().accept(this, p);
      return null;
    }

    @Override
    public @Nullable Void visitDeclared(DeclaredType t, ImmutableSet.Builder<TypeElement> p) {
      p.add(MoreElements.asType(t.asElement()));
      for (TypeMirror typeArgument : t.getTypeArguments()) {
        typeArgument.accept(this, p);
      }
      return null;
    }

    @Override
    public @Nullable Void visitTypeVariable(TypeVariable t, ImmutableSet.Builder<TypeElement> p) {
      t.getLowerBound().accept(this, p);
      t.getUpperBound().accept(this, p);
      return null;
    }

    @Override
    public @Nullable Void visitWildcard(WildcardType t, ImmutableSet.Builder<TypeElement> p) {
      TypeMirror extendsBound = t.getExtendsBound();
      if (extendsBound != null) {
        extendsBound.accept(this, p);
      }
      TypeMirror superBound = t.getSuperBound();
      if (superBound != null) {
        superBound.accept(this, p);
      }
      return null;
    }
  }

  /**
   * An alternate implementation of {@link Types#asElement} that does not require a {@link Types}
   * instance with the notable difference that it will throw {@link IllegalArgumentException}
   * instead of returning null if the {@link TypeMirror} can not be converted to an {@link Element}.
   *
   * @throws NullPointerException if {@code typeMirror} is {@code null}
   * @throws IllegalArgumentException if {@code typeMirror} cannot be converted to an {@link
   *     Element}
   */
  public static Element asElement(TypeMirror typeMirror) {
    return typeMirror.accept(AsElementVisitor.INSTANCE, null);
  }

  private static final class AsElementVisitor extends SimpleTypeVisitor8<Element, Void> {
    private static final AsElementVisitor INSTANCE = new AsElementVisitor();

    @Override
    protected Element defaultAction(TypeMirror e, Void p) {
      throw new IllegalArgumentException(e + " cannot be converted to an Element");
    }

    @Override
    public Element visitDeclared(DeclaredType t, Void p) {
      return t.asElement();
    }

    @Override
    public Element visitError(ErrorType t, Void p) {
      return t.asElement();
    }

    @Override
    public Element visitTypeVariable(TypeVariable t, Void p) {
      return t.asElement();
    }
  }
  ;

  // TODO(gak): consider removing these two methods as they're pretty trivial now
  public static TypeElement asTypeElement(TypeMirror mirror) {
    return MoreElements.asType(asElement(mirror));
  }

  public static ImmutableSet<TypeElement> asTypeElements(Iterable<? extends TypeMirror> mirrors) {
    checkNotNull(mirrors);
    ImmutableSet.Builder<TypeElement> builder = ImmutableSet.builder();
    for (TypeMirror mirror : mirrors) {
      builder.add(asTypeElement(mirror));
    }
    return builder.build();
  }

  /**
   * Returns a {@link ArrayType} if the {@link TypeMirror} represents an array or throws an {@link
   * IllegalArgumentException}.
   */
  public static ArrayType asArray(TypeMirror maybeArrayType) {
    return maybeArrayType.accept(ArrayTypeVisitor.INSTANCE, null);
  }

  private static final class ArrayTypeVisitor extends CastingTypeVisitor<ArrayType> {
    private static final ArrayTypeVisitor INSTANCE = new ArrayTypeVisitor();

    ArrayTypeVisitor() {
      super("array");
    }

    @Override
    public ArrayType visitArray(ArrayType type, Void ignore) {
      return type;
    }
  }

  /**
   * Returns a {@link DeclaredType} if the {@link TypeMirror} represents a declared type such as a
   * class, interface, union/compound, or enum or throws an {@link IllegalArgumentException}.
   */
  public static DeclaredType asDeclared(TypeMirror maybeDeclaredType) {
    return maybeDeclaredType.accept(DeclaredTypeVisitor.INSTANCE, null);
  }

  private static final class DeclaredTypeVisitor extends CastingTypeVisitor<DeclaredType> {
    private static final DeclaredTypeVisitor INSTANCE = new DeclaredTypeVisitor();

    DeclaredTypeVisitor() {
      super("declared type");
    }

    @Override
    public DeclaredType visitDeclared(DeclaredType type, Void ignore) {
      return type;
    }
  }

  /**
   * Returns a {@link ExecutableType} if the {@link TypeMirror} represents an executable type such
   * as may result from missing code, or bad compiles or throws an {@link IllegalArgumentException}.
   */
  public static ErrorType asError(TypeMirror maybeErrorType) {
    return maybeErrorType.accept(ErrorTypeVisitor.INSTANCE, null);
  }

  private static final class ErrorTypeVisitor extends CastingTypeVisitor<ErrorType> {
    private static final ErrorTypeVisitor INSTANCE = new ErrorTypeVisitor();

    ErrorTypeVisitor() {
      super("error type");
    }

    @Override
    public ErrorType visitError(ErrorType type, Void ignore) {
      return type;
    }
  }

  /**
   * Returns a {@link ExecutableType} if the {@link TypeMirror} represents an executable type such
   * as a method, constructor, or initializer or throws an {@link IllegalArgumentException}.
   */
  public static ExecutableType asExecutable(TypeMirror maybeExecutableType) {
    return maybeExecutableType.accept(ExecutableTypeVisitor.INSTANCE, null);
  }

  private static final class ExecutableTypeVisitor extends CastingTypeVisitor<ExecutableType> {
    private static final ExecutableTypeVisitor INSTANCE = new ExecutableTypeVisitor();

    ExecutableTypeVisitor() {
      super("executable type");
    }

    @Override
    public ExecutableType visitExecutable(ExecutableType type, Void ignore) {
      return type;
    }
  }

  /**
   * Returns an {@link IntersectionType} if the {@link TypeMirror} represents an intersection-type
   * or throws an {@link IllegalArgumentException}.
   */
  public static IntersectionType asIntersection(TypeMirror maybeIntersectionType) {
    return maybeIntersectionType.accept(IntersectionTypeVisitor.INSTANCE, null);
  }

  private static final class IntersectionTypeVisitor extends CastingTypeVisitor<IntersectionType> {
    private static final IntersectionTypeVisitor INSTANCE = new IntersectionTypeVisitor();

    IntersectionTypeVisitor() {
      super("intersection type");
    }

    @Override
    public IntersectionType visitIntersection(IntersectionType type, Void ignore) {
      return type;
    }
  }

  /**
   * Returns a {@link NoType} if the {@link TypeMirror} represents an non-type such as void, or
   * package, etc. or throws an {@link IllegalArgumentException}.
   */
  public static NoType asNoType(TypeMirror maybeNoType) {
    return maybeNoType.accept(NoTypeVisitor.INSTANCE, null);
  }

  private static final class NoTypeVisitor extends CastingTypeVisitor<NoType> {
    private static final NoTypeVisitor INSTANCE = new NoTypeVisitor();

    NoTypeVisitor() {
      super("non-type");
    }

    @Override
    public NoType visitNoType(NoType type, Void ignore) {
      return type;
    }
  }

  /**
   * Returns a {@link NullType} if the {@link TypeMirror} represents the null type or throws an
   * {@link IllegalArgumentException}.
   */
  public static NullType asNullType(TypeMirror maybeNullType) {
    return maybeNullType.accept(NullTypeVisitor.INSTANCE, null);
  }

  private static final class NullTypeVisitor extends CastingTypeVisitor<NullType> {
    private static final NullTypeVisitor INSTANCE = new NullTypeVisitor();

    NullTypeVisitor() {
      super("null");
    }

    @Override
    public NullType visitNull(NullType type, Void ignore) {
      return type;
    }
  }

  /**
   * Returns a {@link PrimitiveType} if the {@link TypeMirror} represents a primitive type or throws
   * an {@link IllegalArgumentException}.
   */
  public static PrimitiveType asPrimitiveType(TypeMirror maybePrimitiveType) {
    return maybePrimitiveType.accept(PrimitiveTypeVisitor.INSTANCE, null);
  }

  private static final class PrimitiveTypeVisitor extends CastingTypeVisitor<PrimitiveType> {
    private static final PrimitiveTypeVisitor INSTANCE = new PrimitiveTypeVisitor();

    PrimitiveTypeVisitor() {
      super("primitive type");
    }

    @Override
    public PrimitiveType visitPrimitive(PrimitiveType type, Void ignore) {
      return type;
    }
  }

  //
  // visitUnionType would go here, but isn't relevant for annotation processors
  //

  /**
   * Returns a {@link TypeVariable} if the {@link TypeMirror} represents a type variable or throws
   * an {@link IllegalArgumentException}.
   */
  public static TypeVariable asTypeVariable(TypeMirror maybeTypeVariable) {
    return maybeTypeVariable.accept(TypeVariableVisitor.INSTANCE, null);
  }

  private static final class TypeVariableVisitor extends CastingTypeVisitor<TypeVariable> {
    private static final TypeVariableVisitor INSTANCE = new TypeVariableVisitor();

    TypeVariableVisitor() {
      super("type variable");
    }

    @Override
    public TypeVariable visitTypeVariable(TypeVariable type, Void ignore) {
      return type;
    }
  }

  /**
   * Returns a {@link WildcardType} if the {@link TypeMirror} represents a wildcard type or throws
   * an {@link IllegalArgumentException}.
   */
  public static WildcardType asWildcard(TypeMirror maybeWildcardType) {
    return maybeWildcardType.accept(WildcardTypeVisitor.INSTANCE, null);
  }

  private static final class WildcardTypeVisitor extends CastingTypeVisitor<WildcardType> {
    private static final WildcardTypeVisitor INSTANCE = new WildcardTypeVisitor();

    WildcardTypeVisitor() {
      super("wildcard type");
    }

    @Override
    public WildcardType visitWildcard(WildcardType type, Void ignore) {
      return type;
    }
  }

  /**
   * Returns true if the raw type underlying the given {@link TypeMirror} represents a type that can
   * be referenced by a {@link Class}. If this returns true, then {@link #isTypeOf} is guaranteed to
   * not throw.
   */
  public static boolean isType(TypeMirror type) {
    return type.accept(IsTypeVisitor.INSTANCE, null);
  }

  private static final class IsTypeVisitor extends SimpleTypeVisitor8<Boolean, Void> {
    private static final IsTypeVisitor INSTANCE = new IsTypeVisitor();

    @Override
    protected Boolean defaultAction(TypeMirror type, Void ignored) {
      return false;
    }

    @Override
    public Boolean visitNoType(NoType noType, Void p) {
      return noType.getKind().equals(TypeKind.VOID);
    }

    @Override
    public Boolean visitPrimitive(PrimitiveType type, Void p) {
      return true;
    }

    @Override
    public Boolean visitArray(ArrayType array, Void p) {
      return true;
    }

    @Override
    public Boolean visitDeclared(DeclaredType type, Void ignored) {
      return MoreElements.isType(type.asElement());
    }
  }

  /**
   * Returns true if the raw type underlying the given {@link TypeMirror} represents the same raw
   * type as the given {@link Class} and throws an IllegalArgumentException if the {@link
   * TypeMirror} does not represent a type that can be referenced by a {@link Class}
   */
  public static boolean isTypeOf(final Class<?> clazz, TypeMirror type) {
    checkNotNull(clazz);
    return type.accept(new IsTypeOf(clazz), null);
  }

  private static final class IsTypeOf extends SimpleTypeVisitor8<Boolean, Void> {
    private final Class<?> clazz;

    IsTypeOf(Class<?> clazz) {
      this.clazz = clazz;
    }

    @Override
    protected Boolean defaultAction(TypeMirror type, Void ignored) {
      throw new IllegalArgumentException(type + " cannot be represented as a Class<?>.");
    }

    @Override
    public Boolean visitNoType(NoType noType, Void p) {
      if (noType.getKind().equals(TypeKind.VOID)) {
        return clazz.equals(Void.TYPE);
      }
      throw new IllegalArgumentException(noType + " cannot be represented as a Class<?>.");
    }

    @Override
    public Boolean visitError(ErrorType errorType, Void p) {
      return false;
    }

    @Override
    public Boolean visitPrimitive(PrimitiveType type, Void p) {
      switch (type.getKind()) {
        case BOOLEAN:
          return clazz.equals(Boolean.TYPE);
        case BYTE:
          return clazz.equals(Byte.TYPE);
        case CHAR:
          return clazz.equals(Character.TYPE);
        case DOUBLE:
          return clazz.equals(Double.TYPE);
        case FLOAT:
          return clazz.equals(Float.TYPE);
        case INT:
          return clazz.equals(Integer.TYPE);
        case LONG:
          return clazz.equals(Long.TYPE);
        case SHORT:
          return clazz.equals(Short.TYPE);
        default:
          throw new IllegalArgumentException(type + " cannot be represented as a Class<?>.");
      }
    }

    @Override
    public Boolean visitArray(ArrayType array, Void p) {
      return clazz.isArray() && isTypeOf(clazz.getComponentType(), array.getComponentType());
    }

    @Override
    public Boolean visitDeclared(DeclaredType type, Void ignored) {
      TypeElement typeElement = MoreElements.asType(type.asElement());
      return typeElement.getQualifiedName().contentEquals(clazz.getCanonicalName());
    }
  }

  /**
   * Returns the superclass of {@code type}, with any type parameters bound by {@code type}, or
   * {@link Optional#absent()} if {@code type} is an interface or {@link Object} or its superclass
   * is {@link Object}.
   */
  // TODO(bcorso): Remove unused parameter Elements?
  public static Optional<DeclaredType> nonObjectSuperclass(
      Types types, Elements elements, DeclaredType type) {
    checkNotNull(types);
    checkNotNull(elements); // This is no longer used, but here to avoid changing the API.
    checkNotNull(type);

    TypeMirror superclassType = asTypeElement(type).getSuperclass();
    if (!isType(superclassType)) { // type is Object or an interface
      return Optional.absent();
    }

    DeclaredType superclass = asDeclared(superclassType);
    if (isObjectType(superclass)) {
      return Optional.absent();
    }

    if (superclass.getTypeArguments().isEmpty()) {
      return Optional.of(superclass);
    }

    // In the case where the super class has type parameters, TypeElement#getSuperclass gives
    // SuperClass<T> rather than SuperClass<Foo>, so use Types#directSupertypes instead. The javadoc
    // for Types#directSupertypes guarantees that a super class, if it exists, comes before any
    // interfaces. Thus, we can just get the first element in the list.
    return Optional.of(asDeclared(types.directSupertypes(type).get(0)));
  }

  private static boolean isObjectType(DeclaredType type) {
    return asTypeElement(type).getQualifiedName().contentEquals("java.lang.Object");
  }

  /**
   * Resolves a {@link VariableElement} parameter to a method or constructor based on the given
   * container, or a member of a class. For parameters to a method or constructor, the variable's
   * enclosing element must be a supertype of the container type. For example, given a
   * {@code container} of type {@code Set<String>}, and a variable corresponding to the {@code E e}
   * parameter in the {@code Set.add(E e)} method, this will return a TypeMirror for {@code String}.
   */
  public static TypeMirror asMemberOf(
      Types types, DeclaredType container, VariableElement variable) {
    if (variable.getKind().equals(ElementKind.PARAMETER)) {
      ExecutableElement methodOrConstructor =
          MoreElements.asExecutable(variable.getEnclosingElement());
      ExecutableType resolvedMethodOrConstructor =
          MoreTypes.asExecutable(types.asMemberOf(container, methodOrConstructor));
      List<? extends VariableElement> parameters = methodOrConstructor.getParameters();
      List<? extends TypeMirror> parameterTypes = resolvedMethodOrConstructor.getParameterTypes();
      checkState(parameters.size() == parameterTypes.size());
      for (int i = 0; i < parameters.size(); i++) {
        // We need to capture the parameter type of the variable we're concerned about,
        // for later printing.  This is the only way to do it since we can't use
        // types.asMemberOf on variables of methods.
        if (parameters.get(i).equals(variable)) {
          return parameterTypes.get(i);
        }
      }
      throw new IllegalStateException("Could not find variable: " + variable);
    } else {
      return types.asMemberOf(container, variable);
    }
  }

  private abstract static class CastingTypeVisitor<T> extends SimpleTypeVisitor8<T, Void> {
    private final String label;

    CastingTypeVisitor(String label) {
      this.label = label;
    }

    @Override
    protected T defaultAction(TypeMirror e, Void v) {
      throw new IllegalArgumentException(e + " does not represent a " + label);
    }
  }

  /**
   * Returns true if casting {@code Object} to the given type will elicit an unchecked warning from
   * the compiler. Only type variables and parameterized types such as {@code List<String>} produce
   * such warnings. There will be no warning if the type's only type parameters are simple
   * wildcards, as in {@code Map<?, ?>}.
   */
  public static boolean isConversionFromObjectUnchecked(TypeMirror type) {
    return new CastingUncheckedVisitor().visit(type, null);
  }

  /**
   * Visitor that tells whether a type is erased, in the sense of {@link #castIsUnchecked}. Each
   * visitX method returns true if its input parameter is true or if the type being visited is
   * erased.
   */
  private static class CastingUncheckedVisitor extends SimpleTypeVisitor8<Boolean, Void> {
    CastingUncheckedVisitor() {
      super(false);
    }

    @Override
    public Boolean visitUnknown(TypeMirror t, Void p) {
      // We don't know whether casting is unchecked for this mysterious type but assume it is,
      // so we will insert a possibly unnecessary @SuppressWarnings("unchecked").
      return true;
    }

    @Override
    public Boolean visitArray(ArrayType t, Void p) {
      return visit(t.getComponentType(), p);
    }

    @Override
    public Boolean visitDeclared(DeclaredType t, Void p) {
      return t.getTypeArguments().stream().anyMatch(CastingUncheckedVisitor::uncheckedTypeArgument);
    }

    @Override
    public Boolean visitTypeVariable(TypeVariable t, Void p) {
      return true;
    }

    // If a type has a type argument, then casting to the type is unchecked, except if the argument
    // is <?> or <? extends Object>. The same applies to all type arguments, so casting to Map<?, ?>
    // does not produce an unchecked warning for example.
    private static boolean uncheckedTypeArgument(TypeMirror arg) {
      if (arg.getKind().equals(TypeKind.WILDCARD)) {
        WildcardType wildcard = asWildcard(arg);
        if (wildcard.getExtendsBound() == null || isJavaLangObject(wildcard.getExtendsBound())) {
          // This is <?>, unless there's a super bound, in which case it is <? super Foo> and
          // is erased.
          return (wildcard.getSuperBound() != null);
        }
      }
      return true;
    }

    private static boolean isJavaLangObject(TypeMirror type) {
      if (type.getKind() != TypeKind.DECLARED) {
        return false;
      }
      TypeElement typeElement = asTypeElement(type);
      return typeElement.getQualifiedName().contentEquals("java.lang.Object");
    }
  }

  private MoreTypes() {}
}