/*

 * Copyright (c) 2008, 2021, Oracle and/or its affiliates. All rights reserved.

 * DO NOT ALTER OR REMOVE COPYRIGHT NOTICES OR THIS FILE HEADER.

 *

 * This code is free software; you can redistribute it and/or modify it

 * under the terms of the GNU General Public License version 2 only, as

 * published by the Free Software Foundation.  Oracle designates this

 * particular file as subject to the "Classpath" exception as provided

 * by Oracle in the LICENSE file that accompanied this code.

 *

 * This code is distributed in the hope that it will be useful, but WITHOUT

 * ANY WARRANTY; without even the implied warranty of MERCHANTABILITY or

 * FITNESS FOR A PARTICULAR PURPOSE.  See the GNU General Public License

 * version 2 for more details (a copy is included in the LICENSE file that

 * accompanied this code).

 *

 * You should have received a copy of the GNU General Public License version

 * 2 along with this work; if not, write to the Free Software Foundation,

 * Inc., 51 Franklin St, Fifth Floor, Boston, MA 02110-1301 USA.

 *

 * Please contact Oracle, 500 Oracle Parkway, Redwood Shores, CA 94065 USA

 * or visit www.oracle.com if you need additional information or have any

 * questions.

 */

package java.lang.invoke;

import java.lang.constant.ClassDesc;

import java.lang.constant.Constable;

import java.lang.constant.MethodTypeDesc;

import java.lang.ref.Reference;

import java.lang.ref.ReferenceQueue;

import java.lang.ref.WeakReference;

import java.util.Arrays;

import java.util.Collections;

import java.util.List;

import java.util.NoSuchElementException;

import java.util.Objects;

import java.util.Optional;

import java.util.StringJoiner;

import java.util.concurrent.ConcurrentHashMap;

import java.util.concurrent.ConcurrentMap;

import java.util.stream.Stream;

import jdk.internal.vm.annotation.Stable;

import sun.invoke.util.BytecodeDescriptor;

import sun.invoke.util.VerifyType;

import sun.invoke.util.Wrapper;

import sun.security.util.SecurityConstants;

import static java.lang.invoke.MethodHandleStatics.UNSAFE;

import static java.lang.invoke.MethodHandleStatics.newIllegalArgumentException;

import static java.lang.invoke.MethodType.fromDescriptor;

/**

 * A method type represents the arguments and return type accepted and

 * returned by a method handle, or the arguments and return type passed

 * and expected  by a method handle caller.  Method types must be properly

 * matched between a method handle and all its callers,

 * and the JVM's operations enforce this matching at, specifically

 * during calls to {@link MethodHandle#invokeExact MethodHandle.invokeExact}

 * and {@link MethodHandle#invoke MethodHandle.invoke}, and during execution

 * of {@code invokedynamic} instructions.

 * <p>

 * The structure is a return type accompanied by any number of parameter types.

 * The types (primitive, {@code void}, and reference) are represented by {@link Class} objects.

 * (For ease of exposition, we treat {@code void} as if it were a type.

 * In fact, it denotes the absence of a return type.)

 * <p>

 * All instances of {@code MethodType} are immutable.

 * Two instances are completely interchangeable if they compare equal.

 * Equality depends on pairwise correspondence of the return and parameter types and on nothing else.

 * <p>

 * This type can be created only by factory methods.

 * All factory methods may cache values, though caching is not guaranteed.

 * Some factory methods are static, while others are virtual methods which

 * modify precursor method types, e.g., by changing a selected parameter.

 * <p>

 * Factory methods which operate on groups of parameter types

 * are systematically presented in two versions, so that both Java arrays and

 * Java lists can be used to work with groups of parameter types.

 * The query methods {@code parameterArray} and {@code parameterList}

 * also provide a choice between arrays and lists.

 * <p>

 * {@code MethodType} objects are sometimes derived from bytecode instructions

 * such as {@code invokedynamic}, specifically from the type descriptor strings associated

 * with the instructions in a class file's constant pool.

 * <p>

 * Like classes and strings, method types can also be represented directly

 * in a class file's constant pool as constants.

 * A method type may be loaded by an {@code ldc} instruction which refers

 * to a suitable {@code CONSTANT_MethodType} constant pool entry.

 * The entry refers to a {@code CONSTANT_Utf8} spelling for the descriptor string.

 * (For full details on method type constants, see sections {@jvms

 * 4.4.8} and {@jvms 5.4.3.5} of the Java Virtual Machine

 * Specification.)

 * <p>

 * When the JVM materializes a {@code MethodType} from a descriptor string,

 * all classes named in the descriptor must be accessible, and will be loaded.

 * (But the classes need not be initialized, as is the case with a {@code CONSTANT_Class}.)

 * This loading may occur at any time before the {@code MethodType} object is first derived.

 * <p>

 * <b><a id="descriptor">Nominal Descriptors</a></b>

 * <p>

 * A {@code MethodType} can be described in {@linkplain MethodTypeDesc nominal form}

 * if and only if all of the parameter types and return type can be described

 * with a {@link Class#describeConstable() nominal descriptor} represented by

 * {@link ClassDesc}.  If a method type can be described nominally, then:

 * <ul>

 * <li>The method type has a {@link MethodTypeDesc nominal descriptor}

 *     returned by {@link #describeConstable() MethodType::describeConstable}.</li>

 * <li>The descriptor string returned by

 *     {@link #descriptorString() MethodType::descriptorString} or

 *     {@link #toMethodDescriptorString() MethodType::toMethodDescriptorString}

 *     for the method type is a method descriptor (JVMS {@jvms 4.3.3}).</li>

 * </ul>

 * <p>

 * If any of the parameter types or return type cannot be described

 * nominally, i.e. {@link Class#describeConstable() Class::describeConstable}

 * returns an empty optional for that type,

 * then the method type cannot be described nominally:

 * <ul>

 * <li>The method type has no {@link MethodTypeDesc nominal descriptor} and

 *     {@link #describeConstable() MethodType::describeConstable} returns

 *     an empty optional.</li>

 * <li>The descriptor string returned by

 *     {@link #descriptorString() MethodType::descriptorString} or

 *     {@link #toMethodDescriptorString() MethodType::toMethodDescriptorString}

 *     for the method type is not a type descriptor.</li>

 * </ul>

 *

 * @author John Rose, JSR 292 EG

 * @since 1.7

 */

public final

class MethodType

        implements Constable,

                   TypeDescriptor.OfMethod<Class<?>, MethodType>,

                   java.io.Serializable {

    @java.io.Serial

    private static final long serialVersionUID = 292L;  // {rtype, {ptype...}}

    // The rtype and ptypes fields define the structural identity of the method type:

    private final @Stable Class<?>   rtype;

    private final @Stable Class<?>[] ptypes;

    // The remaining fields are caches of various sorts:

    private @Stable MethodTypeForm form; // erased form, plus cached data about primitives

    private @Stable Object wrapAlt;  // alternative wrapped/unwrapped version and

                                     // private communication for readObject and readResolve

    private @Stable Invokers invokers;   // cache of handy higher-order adapters

    private @Stable String methodDescriptor;  // cache for toMethodDescriptorString

    /**

     * Constructor that performs no copying or validation.

     * Should only be called from the factory method makeImpl

     */

    private MethodType(Class<?> rtype, Class<?>[] ptypes) {

        this.rtype = rtype;

        this.ptypes = ptypes;

    }

    /*trusted*/ MethodTypeForm form() { return form; }

    /*trusted*/ Class<?> rtype() { return rtype; }

    /*trusted*/ Class<?>[] ptypes() { return ptypes; }

    void setForm(MethodTypeForm f) { form = f; }

    /** This number, mandated by the JVM spec as 255,

     *  is the maximum number of <em>slots</em>

     *  that any Java method can receive in its argument list.

     *  It limits both JVM signatures and method type objects.

     *  The longest possible invocation will look like

     *  {@code staticMethod(arg1, arg2, ..., arg255)} or

     *  {@code x.virtualMethod(arg1, arg2, ..., arg254)}.

     */

    /*non-public*/

    static final int MAX_JVM_ARITY = 255;  // this is mandated by the JVM spec.

    /** This number is the maximum arity of a method handle, 254.

     *  It is derived from the absolute JVM-imposed arity by subtracting one,

     *  which is the slot occupied by the method handle itself at the

     *  beginning of the argument list used to invoke the method handle.

     *  The longest possible invocation will look like

     *  {@code mh.invoke(arg1, arg2, ..., arg254)}.

     */

    // Issue:  Should we allow MH.invokeWithArguments to go to the full 255?

    /*non-public*/

    static final int MAX_MH_ARITY = MAX_JVM_ARITY-1;  // deduct one for mh receiver

    /** This number is the maximum arity of a method handle invoker, 253.

     *  It is derived from the absolute JVM-imposed arity by subtracting two,

     *  which are the slots occupied by invoke method handle, and the

     *  target method handle, which are both at the beginning of the argument

     *  list used to invoke the target method handle.

     *  The longest possible invocation will look like

     *  {@code invokermh.invoke(targetmh, arg1, arg2, ..., arg253)}.

     */

    /*non-public*/

    static final int MAX_MH_INVOKER_ARITY = MAX_MH_ARITY-1;  // deduct one more for invoker

    /** Return number of extra slots (count of long/double args). */

    private static int checkPtypes(Class<?>[] ptypes) {

        int slots = 0;

        for (Class<?> ptype : ptypes) {

            Objects.requireNonNull(ptype);

            if (ptype == void.class)

                throw newIllegalArgumentException("parameter type cannot be void");

            if (ptype == double.class || ptype == long.class) {

                slots++;

            }

        }

        checkSlotCount(ptypes.length + slots);

        return slots;

    }

    static {

        // MAX_JVM_ARITY must be power of 2 minus 1 for following code trick to work:

        assert((MAX_JVM_ARITY & (MAX_JVM_ARITY+1)) == 0);

    }

    static void checkSlotCount(int count) {

        if ((count & MAX_JVM_ARITY) != count)

            throw newIllegalArgumentException("bad parameter count "+count);

    }

    private static IndexOutOfBoundsException newIndexOutOfBoundsException(Object num) {

        if (num instanceof Integer)  num = "bad index: "+num;

        return new IndexOutOfBoundsException(num.toString());

    }

    static final ConcurrentWeakInternSet<MethodType> internTable = new ConcurrentWeakInternSet<>();

    static final Class<?>[] NO_PTYPES = {};

    /**

     * Finds or creates an instance of the given method type.

     * @param rtype  the return type

     * @param ptypes the parameter types

     * @return a method type with the given components

     * @throws NullPointerException if {@code rtype} or {@code ptypes} or any element of {@code ptypes} is null

     * @throws IllegalArgumentException if any element of {@code ptypes} is {@code void.class}

     */

    public static MethodType methodType(Class<?> rtype, Class<?>[] ptypes) {

        return makeImpl(rtype, ptypes, false);

    }

    /**

     * Finds or creates a method type with the given components.

     * Convenience method for {@link #methodType(java.lang.Class, java.lang.Class[]) methodType}.

     * @param rtype  the return type

     * @param ptypes the parameter types

     * @return a method type with the given components

     * @throws NullPointerException if {@code rtype} or {@code ptypes} or any element of {@code ptypes} is null

     * @throws IllegalArgumentException if any element of {@code ptypes} is {@code void.class}

     */

    public static MethodType methodType(Class<?> rtype, List<Class<?>> ptypes) {

        boolean notrust = false;  // random List impl. could return evil ptypes array

        return makeImpl(rtype, listToArray(ptypes), notrust);

    }

    private static Class<?>[] listToArray(List<Class<?>> ptypes) {

        // sanity check the size before the toArray call, since size might be huge

        checkSlotCount(ptypes.size());

        return ptypes.toArray(NO_PTYPES);

    }

    /**

     * Finds or creates a method type with the given components.

     * Convenience method for {@link #methodType(java.lang.Class, java.lang.Class[]) methodType}.

     * The leading parameter type is prepended to the remaining array.

     * @param rtype  the return type

     * @param ptype0 the first parameter type

     * @param ptypes the remaining parameter types

     * @return a method type with the given components

     * @throws NullPointerException if {@code rtype} or {@code ptype0} or {@code ptypes} or any element of {@code ptypes} is null

     * @throws IllegalArgumentException if {@code ptype0} or {@code ptypes} or any element of {@code ptypes} is {@code void.class}

     */

    public static MethodType methodType(Class<?> rtype, Class<?> ptype0, Class<?>... ptypes) {

        Class<?>[] ptypes1 = new Class<?>[1+ptypes.length];

        ptypes1[0] = ptype0;

        System.arraycopy(ptypes, 0, ptypes1, 1, ptypes.length);

        return makeImpl(rtype, ptypes1, true);

    }

    /**

     * Finds or creates a method type with the given components.

     * Convenience method for {@link #methodType(java.lang.Class, java.lang.Class[]) methodType}.

     * The resulting method has no parameter types.

     * @param rtype  the return type

     * @return a method type with the given return value

     * @throws NullPointerException if {@code rtype} is null

     */

    public static MethodType methodType(Class<?> rtype) {

        return makeImpl(rtype, NO_PTYPES, true);

    }

    /**

     * Finds or creates a method type with the given components.

     * Convenience method for {@link #methodType(java.lang.Class, java.lang.Class[]) methodType}.

     * The resulting method has the single given parameter type.

     * @param rtype  the return type

     * @param ptype0 the parameter type

     * @return a method type with the given return value and parameter type

     * @throws NullPointerException if {@code rtype} or {@code ptype0} is null

     * @throws IllegalArgumentException if {@code ptype0} is {@code void.class}

     */

    public static MethodType methodType(Class<?> rtype, Class<?> ptype0) {

        return makeImpl(rtype, new Class<?>[]{ ptype0 }, true);

    }

    /**

     * Finds or creates a method type with the given components.

     * Convenience method for {@link #methodType(java.lang.Class, java.lang.Class[]) methodType}.

     * The resulting method has the same parameter types as {@code ptypes},

     * and the specified return type.

     * @param rtype  the return type

     * @param ptypes the method type which supplies the parameter types

     * @return a method type with the given components

     * @throws NullPointerException if {@code rtype} or {@code ptypes} is null

     */

    public static MethodType methodType(Class<?> rtype, MethodType ptypes) {

        return makeImpl(rtype, ptypes.ptypes, true);

    }

    /**

     * Sole factory method to find or create an interned method type. Will perform

     * input validation on behalf of factory methods

     *

     * @param rtype desired return type

     * @param ptypes desired parameter types

     * @param trusted whether the ptypes can be used without cloning

     * @throws NullPointerException if {@code rtype} or {@code ptypes} or any element of {@code ptypes} is null

     * @throws IllegalArgumentException if any element of {@code ptypes} is {@code void.class}

     * @return the unique method type of the desired structure

     */

    /*trusted*/

    static MethodType makeImpl(Class<?> rtype, Class<?>[] ptypes, boolean trusted) {

        if (ptypes.length == 0) {

            ptypes = NO_PTYPES; trusted = true;

        }

        MethodType primordialMT = new MethodType(rtype, ptypes);

        MethodType mt = internTable.get(primordialMT);

        if (mt != null)

            return mt;

        // promote the object to the Real Thing, and reprobe

        Objects.requireNonNull(rtype);

        if (trusted) {

            MethodType.checkPtypes(ptypes);

            mt = primordialMT;

        } else {

            // Make defensive copy then validate

            ptypes = Arrays.copyOf(ptypes, ptypes.length);

            MethodType.checkPtypes(ptypes);

            mt = new MethodType(rtype, ptypes);

        }

        mt.form = MethodTypeForm.findForm(mt);

        return internTable.add(mt);

    }

    private static final @Stable MethodType[] objectOnlyTypes = new MethodType[20];

    /**

     * Finds or creates a method type whose components are {@code Object} with an optional trailing {@code Object[]} array.

     * Convenience method for {@link #methodType(java.lang.Class, java.lang.Class[]) methodType}.

     * All parameters and the return type will be {@code Object},

     * except the final array parameter if any, which will be {@code Object[]}.

     * @param objectArgCount number of parameters (excluding the final array parameter if any)

     * @param finalArray whether there will be a trailing array parameter, of type {@code Object[]}

     * @return a generally applicable method type, for all calls of the given fixed argument count and a collected array of further arguments

     * @throws IllegalArgumentException if {@code objectArgCount} is negative or greater than 255 (or 254, if {@code finalArray} is true)

     * @see #genericMethodType(int)

     */

    public static MethodType genericMethodType(int objectArgCount, boolean finalArray) {

        MethodType mt;

        checkSlotCount(objectArgCount);

        int ivarargs = (!finalArray ? 0 : 1);

        int ootIndex = objectArgCount*2 + ivarargs;

        if (ootIndex < objectOnlyTypes.length) {

            mt = objectOnlyTypes[ootIndex];

            if (mt != null)  return mt;

        }

        Class<?>[] ptypes = new Class<?>[objectArgCount + ivarargs];

        Arrays.fill(ptypes, Object.class);

        if (ivarargs != 0)  ptypes[objectArgCount] = Object[].class;

        mt = makeImpl(Object.class, ptypes, true);

        if (ootIndex < objectOnlyTypes.length) {

            objectOnlyTypes[ootIndex] = mt;     // cache it here also!

        }

        return mt;

    }

    /**

     * Finds or creates a method type whose components are all {@code Object}.

     * Convenience method for {@link #methodType(java.lang.Class, java.lang.Class[]) methodType}.

     * All parameters and the return type will be Object.

     * @param objectArgCount number of parameters

     * @return a generally applicable method type, for all calls of the given argument count

     * @throws IllegalArgumentException if {@code objectArgCount} is negative or greater than 255

     * @see #genericMethodType(int, boolean)

     */

    public static MethodType genericMethodType(int objectArgCount) {

        return genericMethodType(objectArgCount, false);

    }

    /**

     * Finds or creates a method type with a single different parameter type.

     * Convenience method for {@link #methodType(java.lang.Class, java.lang.Class[]) methodType}.

     * @param num    the index (zero-based) of the parameter type to change

     * @param nptype a new parameter type to replace the old one with

     * @return the same type, except with the selected parameter changed

     * @throws IndexOutOfBoundsException if {@code num} is not a valid index into {@code parameterArray()}

     * @throws IllegalArgumentException if {@code nptype} is {@code void.class}

     * @throws NullPointerException if {@code nptype} is null

     */

    public MethodType changeParameterType(int num, Class<?> nptype) {

        if (parameterType(num) == nptype)  return this;

        Class<?>[] nptypes = ptypes.clone();

        nptypes[num] = nptype;

        return makeImpl(rtype, nptypes, true);

    }

    /**

     * Finds or creates a method type with additional parameter types.

     * Convenience method for {@link #methodType(java.lang.Class, java.lang.Class[]) methodType}.

     * @param num    the position (zero-based) of the inserted parameter type(s)

     * @param ptypesToInsert zero or more new parameter types to insert into the parameter list

     * @return the same type, except with the selected parameter(s) inserted

     * @throws IndexOutOfBoundsException if {@code num} is negative or greater than {@code parameterCount()}

     * @throws IllegalArgumentException if any element of {@code ptypesToInsert} is {@code void.class}

     *                                  or if the resulting method type would have more than 255 parameter slots

     * @throws NullPointerException if {@code ptypesToInsert} or any of its elements is null

     */

    public MethodType insertParameterTypes(int num, Class<?>... ptypesToInsert) {

        int len = ptypes.length;

        if (num < 0 || num > len)

            throw newIndexOutOfBoundsException(num);

        int ins = checkPtypes(ptypesToInsert);

        checkSlotCount(parameterSlotCount() + ptypesToInsert.length + ins);

        int ilen = ptypesToInsert.length;

        if (ilen == 0)  return this;

        Class<?>[] nptypes = new Class<?>[len + ilen];

        if (num > 0) {

            System.arraycopy(ptypes, 0, nptypes, 0, num);

        }

        System.arraycopy(ptypesToInsert, 0, nptypes, num, ilen);

        if (num < len) {

            System.arraycopy(ptypes, num, nptypes, num+ilen, len-num);

        }

        return makeImpl(rtype, nptypes, true);

    }

    /**

     * Finds or creates a method type with additional parameter types.

     * Convenience method for {@link #methodType(java.lang.Class, java.lang.Class[]) methodType}.

     * @param ptypesToInsert zero or more new parameter types to insert after the end of the parameter list

     * @return the same type, except with the selected parameter(s) appended

     * @throws IllegalArgumentException if any element of {@code ptypesToInsert} is {@code void.class}

     *                                  or if the resulting method type would have more than 255 parameter slots

     * @throws NullPointerException if {@code ptypesToInsert} or any of its elements is null

     */

    public MethodType appendParameterTypes(Class<?>... ptypesToInsert) {

        return insertParameterTypes(parameterCount(), ptypesToInsert);

    }

    /**

     * Finds or creates a method type with additional parameter types.

     * Convenience method for {@link #methodType(java.lang.Class, java.lang.Class[]) methodType}.

     * @param num    the position (zero-based) of the inserted parameter type(s)

     * @param ptypesToInsert zero or more new parameter types to insert into the parameter list

     * @return the same type, except with the selected parameter(s) inserted

     * @throws IndexOutOfBoundsException if {@code num} is negative or greater than {@code parameterCount()}

     * @throws IllegalArgumentException if any element of {@code ptypesToInsert} is {@code void.class}

     *                                  or if the resulting method type would have more than 255 parameter slots

     * @throws NullPointerException if {@code ptypesToInsert} or any of its elements is null

     */

    public MethodType insertParameterTypes(int num, List<Class<?>> ptypesToInsert) {

        return insertParameterTypes(num, listToArray(ptypesToInsert));

    }

    /**

     * Finds or creates a method type with additional parameter types.

     * Convenience method for {@link #methodType(java.lang.Class, java.lang.Class[]) methodType}.

     * @param ptypesToInsert zero or more new parameter types to insert after the end of the parameter list

     * @return the same type, except with the selected parameter(s) appended

     * @throws IllegalArgumentException if any element of {@code ptypesToInsert} is {@code void.class}

     *                                  or if the resulting method type would have more than 255 parameter slots

     * @throws NullPointerException if {@code ptypesToInsert} or any of its elements is null

     */

    public MethodType appendParameterTypes(List<Class<?>> ptypesToInsert) {

        return insertParameterTypes(parameterCount(), ptypesToInsert);

    }

    /**

     * Finds or creates a method type with modified parameter types.

     * Convenience method for {@link #methodType(java.lang.Class, java.lang.Class[]) methodType}.

     * @param start  the position (zero-based) of the first replaced parameter type(s)

     * @param end    the position (zero-based) after the last replaced parameter type(s)

     * @param ptypesToInsert zero or more new parameter types to insert into the parameter list

     * @return the same type, except with the selected parameter(s) replaced

     * @throws IndexOutOfBoundsException if {@code start} is negative or greater than {@code parameterCount()}

     *                                  or if {@code end} is negative or greater than {@code parameterCount()}

     *                                  or if {@code start} is greater than {@code end}

     * @throws IllegalArgumentException if any element of {@code ptypesToInsert} is {@code void.class}

     *                                  or if the resulting method type would have more than 255 parameter slots

     * @throws NullPointerException if {@code ptypesToInsert} or any of its elements is null

     */

    /*non-public*/

    MethodType replaceParameterTypes(int start, int end, Class<?>... ptypesToInsert) {

        if (start == end)

            return insertParameterTypes(start, ptypesToInsert);

        int len = ptypes.length;

        if (!(0 <= start && start <= end && end <= len))

            throw newIndexOutOfBoundsException("start="+start+" end="+end);

        int ilen = ptypesToInsert.length;

        if (ilen == 0)

            return dropParameterTypes(start, end);

        return dropParameterTypes(start, end).insertParameterTypes(start, ptypesToInsert);

    }

    /** Replace the last arrayLength parameter types with the component type of arrayType.

     * @param arrayType any array type

     * @param pos position at which to spread

     * @param arrayLength the number of parameter types to change

     * @return the resulting type

     */

    /*non-public*/

    MethodType asSpreaderType(Class<?> arrayType, int pos, int arrayLength) {

        assert(parameterCount() >= arrayLength);

        int spreadPos = pos;

        if (arrayLength == 0)  return this;  // nothing to change

        if (arrayType == Object[].class) {

            if (isGeneric())  return this;  // nothing to change

            if (spreadPos == 0) {

                // no leading arguments to preserve; go generic

                MethodType res = genericMethodType(arrayLength);

                if (rtype != Object.class) {

                    res = res.changeReturnType(rtype);

                }

                return res;

            }

        }

        Class<?> elemType = arrayType.getComponentType();

        assert(elemType != null);

        for (int i = spreadPos; i < spreadPos + arrayLength; i++) {

            if (ptypes[i] != elemType) {

                Class<?>[] fixedPtypes = ptypes.clone();

                Arrays.fill(fixedPtypes, i, spreadPos + arrayLength, elemType);

                return methodType(rtype, fixedPtypes);

            }

        }

        return this;  // arguments check out; no change

    }

    /** Return the leading parameter type, which must exist and be a reference.

     *  @return the leading parameter type, after error checks

     */

    /*non-public*/

    Class<?> leadingReferenceParameter() {

        Class<?> ptype;

        if (ptypes.length == 0 ||

            (ptype = ptypes[0]).isPrimitive())

            throw newIllegalArgumentException("no leading reference parameter");

        return ptype;

    }

    /** Delete the last parameter type and replace it with arrayLength copies of the component type of arrayType.

     * @param arrayType any array type

     * @param pos position at which to insert parameters

     * @param arrayLength the number of parameter types to insert

     * @return the resulting type

     */

    /*non-public*/

    MethodType asCollectorType(Class<?> arrayType, int pos, int arrayLength) {

        assert(parameterCount() >= 1);

        assert(pos < ptypes.length);

        assert(ptypes[pos].isAssignableFrom(arrayType));

        MethodType res;

        if (arrayType == Object[].class) {

            res = genericMethodType(arrayLength);

            if (rtype != Object.class) {

                res = res.changeReturnType(rtype);

            }

        } else {

            Class<?> elemType = arrayType.getComponentType();

            assert(elemType != null);

            res = methodType(rtype, Collections.nCopies(arrayLength, elemType));

        }

        if (ptypes.length == 1) {

            return res;

        } else {

            // insert after (if need be), then before

            if (pos < ptypes.length - 1) {

                res = res.insertParameterTypes(arrayLength, Arrays.copyOfRange(ptypes, pos + 1, ptypes.length));

            }

            return res.insertParameterTypes(0, Arrays.copyOf(ptypes, pos));

        }

    }

    /**

     * Finds or creates a method type with some parameter types omitted.

     * Convenience method for {@link #methodType(java.lang.Class, java.lang.Class[]) methodType}.

     * @param start  the index (zero-based) of the first parameter type to remove

     * @param end    the index (greater than {@code start}) of the first parameter type after not to remove

     * @return the same type, except with the selected parameter(s) removed

     * @throws IndexOutOfBoundsException if {@code start} is negative or greater than {@code parameterCount()}

     *                                  or if {@code end} is negative or greater than {@code parameterCount()}

     *                                  or if {@code start} is greater than {@code end}

     */

    public MethodType dropParameterTypes(int start, int end) {

        int len = ptypes.length;

        if (!(0 <= start && start <= end && end <= len))

            throw newIndexOutOfBoundsException("start="+start+" end="+end);

        if (start == end)  return this;

        Class<?>[] nptypes;

        if (start == 0) {

            if (end == len) {

                // drop all parameters

                nptypes = NO_PTYPES;

            } else {

                // drop initial parameter(s)

                nptypes = Arrays.copyOfRange(ptypes, end, len);

            }

        } else {

            if (end == len) {

                // drop trailing parameter(s)

                nptypes = Arrays.copyOfRange(ptypes, 0, start);

            } else {

                int tail = len - end;

                nptypes = Arrays.copyOfRange(ptypes, 0, start + tail);

                System.arraycopy(ptypes, end, nptypes, start, tail);

            }

        }

        return makeImpl(rtype, nptypes, true);

    }

    /**

     * Finds or creates a method type with a different return type.

     * Convenience method for {@link #methodType(java.lang.Class, java.lang.Class[]) methodType}.

     * @param nrtype a return parameter type to replace the old one with

     * @return the same type, except with the return type change

     * @throws NullPointerException if {@code nrtype} is null

     */

    public MethodType changeReturnType(Class<?> nrtype) {

        if (returnType() == nrtype)  return this;

        return makeImpl(nrtype, ptypes, true);

    }

    /**

     * Reports if this type contains a primitive argument or return value.

     * The return type {@code void} counts as a primitive.

     * @return true if any of the types are primitives

     */

    public boolean hasPrimitives() {

        return form.hasPrimitives();

    }

    /**

     * Reports if this type contains a wrapper argument or return value.

     * Wrappers are types which box primitive values, such as {@link Integer}.

     * The reference type {@code java.lang.Void} counts as a wrapper,

     * if it occurs as a return type.

     * @return true if any of the types are wrappers

     */

    public boolean hasWrappers() {

        return unwrap() != this;

    }

    /**

     * Erases all reference types to {@code Object}.

     * Convenience method for {@link #methodType(java.lang.Class, java.lang.Class[]) methodType}.

     * All primitive types (including {@code void}) will remain unchanged.

     * @return a version of the original type with all reference types replaced

     */

    public MethodType erase() {

        return form.erasedType();

    }

    /**

     * Erases all reference types to {@code Object}, and all subword types to {@code int}.

     * This is the reduced type polymorphism used by private methods

     * such as {@link MethodHandle#invokeBasic invokeBasic}.

     * @return a version of the original type with all reference and subword types replaced

     */

    /*non-public*/

    MethodType basicType() {

        return form.basicType();

    }

    private static final @Stable Class<?>[] METHOD_HANDLE_ARRAY

            = new Class<?>[] { MethodHandle.class };

    /**

     * @return a version of the original type with MethodHandle prepended as the first argument

     */

    /*non-public*/

    MethodType invokerType() {

        return insertParameterTypes(0, METHOD_HANDLE_ARRAY);

    }

    /**

     * Converts all types, both reference and primitive, to {@code Object}.

     * Convenience method for {@link #genericMethodType(int) genericMethodType}.

     * The expression {@code type.wrap().erase()} produces the same value

     * as {@code type.generic()}.

     * @return a version of the original type with all types replaced

     */

    public MethodType generic() {

        return genericMethodType(parameterCount());

    }

    /*non-public*/

    boolean isGeneric() {

        return this == erase() && !hasPrimitives();

    }

    /**

     * Converts all primitive types to their corresponding wrapper types.

     * Convenience method for {@link #methodType(java.lang.Class, java.lang.Class[]) methodType}.

     * All reference types (including wrapper types) will remain unchanged.

     * A {@code void} return type is changed to the type {@code java.lang.Void}.

     * The expression {@code type.wrap().erase()} produces the same value

     * as {@code type.generic()}.

     * @return a version of the original type with all primitive types replaced

     */

    public MethodType wrap() {

        return hasPrimitives() ? wrapWithPrims(this) : this;

    }

    /**

     * Converts all wrapper types to their corresponding primitive types.

     * Convenience method for {@link #methodType(java.lang.Class, java.lang.Class[]) methodType}.

     * All primitive types (including {@code void}) will remain unchanged.

     * A return type of {@code java.lang.Void} is changed to {@code void}.

     * @return a version of the original type with all wrapper types replaced

     */

    public MethodType unwrap() {

        MethodType noprims = !hasPrimitives() ? this : wrapWithPrims(this);

        return unwrapWithNoPrims(noprims);

    }

    private static MethodType wrapWithPrims(MethodType pt) {

        assert(pt.hasPrimitives());

        MethodType wt = (MethodType)pt.wrapAlt;

        if (wt == null) {

            // fill in lazily

            wt = MethodTypeForm.canonicalize(pt, MethodTypeForm.WRAP);

            assert(wt != null);

            pt.wrapAlt = wt;

        }

        return wt;

    }

    private static MethodType unwrapWithNoPrims(MethodType wt) {

        assert(!wt.hasPrimitives());

        MethodType uwt = (MethodType)wt.wrapAlt;

        if (uwt == null) {

            // fill in lazily

            uwt = MethodTypeForm.canonicalize(wt, MethodTypeForm.UNWRAP);

            if (uwt == null)

                uwt = wt;    // type has no wrappers or prims at all

            wt.wrapAlt = uwt;

        }

        return uwt;

    }

    /**

     * Returns the parameter type at the specified index, within this method type.

     * @param num the index (zero-based) of the desired parameter type

     * @return the selected parameter type

     * @throws IndexOutOfBoundsException if {@code num} is not a valid index into {@code parameterArray()}

     */

    public Class<?> parameterType(int num) {

        return ptypes[num];

    }

    /**

     * Returns the number of parameter types in this method type.

     * @return the number of parameter types

     */

    public int parameterCount() {

        return ptypes.length;

    }

    /**

     * Returns the return type of this method type.

     * @return the return type

     */

    public Class<?> returnType() {

        return rtype;

    }

    /**

     * Presents the parameter types as a list (a convenience method).

     * The list will be immutable.

     * @return the parameter types (as an immutable list)

     */

    public List<Class<?>> parameterList() {

        return Collections.unmodifiableList(Arrays.asList(ptypes.clone()));

    }

    /**

     * Returns the last parameter type of this method type.

     * If this type has no parameters, the sentinel value

     * {@code void.class} is returned instead.

     * @apiNote

     * <p>

     * The sentinel value is chosen so that reflective queries can be

     * made directly against the result value.

     * The sentinel value cannot be confused with a real parameter,

     * since {@code void} is never acceptable as a parameter type.

     * For variable arity invocation modes, the expression

     * {@link Class#getComponentType lastParameterType().getComponentType()}

     * is useful to query the type of the "varargs" parameter.

     * @return the last parameter type if any, else {@code void.class}

     * @since 10

     */

    public Class<?> lastParameterType() {

        int len = ptypes.length;

        return len == 0 ? void.class : ptypes[len-1];

    }

    /**

     * Presents the parameter types as an array (a convenience method).

     * Changes to the array will not result in changes to the type.

     * @return the parameter types (as a fresh copy if necessary)

     */

    public Class<?>[] parameterArray() {

        return ptypes.clone();

    }

    /**

     * Compares the specified object with this type for equality.

     * That is, it returns {@code true} if and only if the specified object

     * is also a method type with exactly the same parameters and return type.

     * @param x object to compare

     * @see Object#equals(Object)

     */

    // This implementation may also return true if x is a WeakEntry containing

    // a method type that is equal to this. This is an internal implementation

    // detail to allow for faster method type lookups.

    // See ConcurrentWeakInternSet.WeakEntry#equals(Object)

    @Override

    public boolean equals(Object x) {

        if (this == x) {

            return true;

        }

        if (x instanceof MethodType) {

            return equals((MethodType)x);

        }

        if (x instanceof ConcurrentWeakInternSet.WeakEntry) {

            Object o = ((ConcurrentWeakInternSet.WeakEntry)x).get();

            if (o instanceof MethodType) {

                return equals((MethodType)o);

            }

        }

        return false;

    }

    private boolean equals(MethodType that) {

        return this.rtype == that.rtype

            && Arrays.equals(this.ptypes, that.ptypes);

    }

    /**

     * Returns the hash code value for this method type.

     * It is defined to be the same as the hashcode of a List

     * whose elements are the return type followed by the

     * parameter types.

     * @return the hash code value for this method type

     * @see Object#hashCode()

     * @see #equals(Object)

     * @see List#hashCode()

     */

    @Override

    public int hashCode() {

        int hashCode = 31 + rtype.hashCode();

        for (Class<?> ptype : ptypes)

            hashCode = 31 * hashCode + ptype.hashCode();

        return hashCode;

    }

    /**

     * Returns a string representation of the method type,

     * of the form {@code "(PT0,PT1...)RT"}.

     * The string representation of a method type is a

     * parenthesis enclosed, comma separated list of type names,

     * followed immediately by the return type.

     * <p>

     * Each type is represented by its

     * {@link java.lang.Class#getSimpleName simple name}.

     */

    @Override

    public String toString() {

        StringJoiner sj = new StringJoiner(",", "(",

                ")" + rtype.getSimpleName());

        for (int i = 0; i < ptypes.length; i++) {

            sj.add(ptypes[i].getSimpleName());

        }

        return sj.toString();

    }

    /** True if my parameter list is effectively identical to the given full list,

     *  after skipping the given number of my own initial parameters.

     *  In other words, after disregarding {@code skipPos} parameters,

     *  my remaining parameter list is no longer than the {@code fullList}, and

     *  is equal to the same-length initial sublist of {@code fullList}.

     */

    /*non-public*/

    boolean effectivelyIdenticalParameters(int skipPos, List<Class<?>> fullList) {

        int myLen = ptypes.length, fullLen = fullList.size();

        if (skipPos > myLen || myLen - skipPos > fullLen)

            return false;

        List<Class<?>> myList = Arrays.asList(ptypes);

        if (skipPos != 0) {

            myList = myList.subList(skipPos, myLen);

            myLen -= skipPos;

        }

        if (fullLen == myLen)

            return myList.equals(fullList);

        else

            return myList.equals(fullList.subList(0, myLen));

    }

    /** True if the old return type can always be viewed (w/o casting) under new return type,

     *  and the new parameters can be viewed (w/o casting) under the old parameter types.

     */

    /*non-public*/

    boolean isViewableAs(MethodType newType, boolean keepInterfaces) {

        if (!VerifyType.isNullConversion(returnType(), newType.returnType(), keepInterfaces))

            return false;

        if (form == newType.form && form.erasedType == this)

            return true;  // my reference parameters are all Object

        if (ptypes == newType.ptypes)

            return true;

        int argc = parameterCount();

        if (argc != newType.parameterCount())

            return false;

        for (int i = 0; i < argc; i++) {

            if (!VerifyType.isNullConversion(newType.parameterType(i), parameterType(i), keepInterfaces))

                return false;

        }

        return true;

    }

    /*non-public*/

    boolean isConvertibleTo(MethodType newType) {

        MethodTypeForm oldForm = this.form();

        MethodTypeForm newForm = newType.form();

        if (oldForm == newForm)

            // same parameter count, same primitive/object mix

            return true;

        if (!canConvert(returnType(), newType.returnType()))

            return false;

        Class<?>[] srcTypes = newType.ptypes;

        Class<?>[] dstTypes = ptypes;

        if (srcTypes == dstTypes)

            return true;

        int argc;

        if ((argc = srcTypes.length) != dstTypes.length)

            return false;

        if (argc <= 1) {

            if (argc == 1 && !canConvert(srcTypes[0], dstTypes[0]))

                return false;

            return true;

        }

        if ((!oldForm.hasPrimitives() && oldForm.erasedType == this) ||

            (!newForm.hasPrimitives() && newForm.erasedType == newType)) {

            // Somewhat complicated test to avoid a loop of 2 or more trips.

            // If either type has only Object parameters, we know we can convert.

            assert(canConvertParameters(srcTypes, dstTypes));

            return true;

        }

        return canConvertParameters(srcTypes, dstTypes);

    }

    /** Returns true if MHs.explicitCastArguments produces the same result as MH.asType.

     *  If the type conversion is impossible for either, the result should be false.

     */

    /*non-public*/

    boolean explicitCastEquivalentToAsType(MethodType newType) {

        if (this == newType)  return true;

        if (!explicitCastEquivalentToAsType(rtype, newType.rtype)) {

            return false;

        }

        Class<?>[] srcTypes = newType.ptypes;

        Class<?>[] dstTypes = ptypes;

        if (dstTypes == srcTypes) {

            return true;

        }

        assert(dstTypes.length == srcTypes.length);

        for (int i = 0; i < dstTypes.length; i++) {

            if (!explicitCastEquivalentToAsType(srcTypes[i], dstTypes[i])) {

                return false;

            }

        }

        return true;

    }

    /** Reports true if the src can be converted to the dst, by both asType and MHs.eCE,

     *  and with the same effect.

     *  MHs.eCA has the following "upgrades" to MH.asType:

     *  1. interfaces are unchecked (that is, treated as if aliased to Object)

     *     Therefore, {@code Object->CharSequence} is possible in both cases but has different semantics

     *  2. the full matrix of primitive-to-primitive conversions is supported

     *     Narrowing like {@code long->byte} and basic-typing like {@code boolean->int}

     *     are not supported by asType, but anything supported by asType is equivalent

     *     with MHs.eCE.

     *  3a. unboxing conversions can be followed by the full matrix of primitive conversions

     *  3b. unboxing of null is permitted (creates a zero primitive value)

     * Other than interfaces, reference-to-reference conversions are the same.

     * Boxing primitives to references is the same for both operators.

     */

    private static boolean explicitCastEquivalentToAsType(Class<?> src, Class<?> dst) {

        if (src == dst || dst == Object.class || dst == void.class)  return true;

        if (src.isPrimitive()) {

            // Could be a prim/prim conversion, where casting is a strict superset.

            // Or a boxing conversion, which is always to an exact wrapper class.

            return canConvert(src, dst);

        } else if (dst.isPrimitive()) {

            // Unboxing behavior is different between MHs.eCA & MH.asType (see 3b).

            return false;

        } else {

            // R->R always works, but we have to avoid a check-cast to an interface.

            return !dst.isInterface() || dst.isAssignableFrom(src);

        }

    }

    private boolean canConvertParameters(Class<?>[] srcTypes, Class<?>[] dstTypes) {

        for (int i = 0; i < srcTypes.length; i++) {

            if (!canConvert(srcTypes[i], dstTypes[i])) {

                return false;

            }

        }

        return true;

    }

    /*non-public*/

    static boolean canConvert(Class<?> src, Class<?> dst) {

        // short-circuit a few cases:

        if (src == dst || src == Object.class || dst == Object.class)  return true;

        // the remainder of this logic is documented in MethodHandle.asType

        if (src.isPrimitive()) {

            // can force void to an explicit null, a la reflect.Method.invoke

            // can also force void to a primitive zero, by analogy

            if (src == void.class)  return true;  //or !dst.isPrimitive()?

            Wrapper sw = Wrapper.forPrimitiveType(src);

            if (dst.isPrimitive()) {

                // P->P must widen

                return Wrapper.forPrimitiveType(dst).isConvertibleFrom(sw);

            } else {

                // P->R must box and widen

                return dst.isAssignableFrom(sw.wrapperType());

            }

        } else if (dst.isPrimitive()) {

            // any value can be dropped

            if (dst == void.class)  return true;

            Wrapper dw = Wrapper.forPrimitiveType(dst);

            // R->P must be able to unbox (from a dynamically chosen type) and widen

            // For example:

            //   Byte/Number/Comparable/Object -> dw:Byte -> byte.

            //   Character/Comparable/Object -> dw:Character -> char

            //   Boolean/Comparable/Object -> dw:Boolean -> boolean

            // This means that dw must be cast-compatible with src.

            if (src.isAssignableFrom(dw.wrapperType())) {

                return true;

            }

            // The above does not work if the source reference is strongly typed

            // to a wrapper whose primitive must be widened.  For example:

            //   Byte -> unbox:byte -> short/int/long/float/double

            //   Character -> unbox:char -> int/long/float/double

            if (Wrapper.isWrapperType(src) &&

                dw.isConvertibleFrom(Wrapper.forWrapperType(src))) {

                // can unbox from src and then widen to dst

                return true;

            }

            // We have already covered cases which arise due to runtime unboxing

            // of a reference type which covers several wrapper types:

            //   Object -> cast:Integer -> unbox:int -> long/float/double

            //   Serializable -> cast:Byte -> unbox:byte -> byte/short/int/long/float/double

            // An marginal case is Number -> dw:Character -> char, which would be OK if there were a

            // subclass of Number which wraps a value that can convert to char.

            // Since there is none, we don't need an extra check here to cover char or boolean.

            return false;

        } else {

            // R->R always works, since null is always valid dynamically

            return true;

        }

    }

    /// Queries which have to do with the bytecode architecture

    /** Reports the number of JVM stack slots required to invoke a method

     * of this type.  Note that (for historical reasons) the JVM requires

     * a second stack slot to pass long and double arguments.

     * So this method returns {@link #parameterCount() parameterCount} plus the

     * number of long and double parameters (if any).

     * <p>

     * This method is included for the benefit of applications that must

     * generate bytecodes that process method handles and invokedynamic.

     * @return the number of JVM stack slots for this type's parameters

     */

    /*non-public*/

    int parameterSlotCount() {

        return form.parameterSlotCount();

    }

    /*non-public*/

    Invokers invokers() {

        Invokers inv = invokers;

        if (inv != null)  return inv;

        invokers = inv = new Invokers(this);

        return inv;

    }

    /**

     * Finds or creates an instance of a method type, given the spelling of its bytecode descriptor.

     * Convenience method for {@link #methodType(java.lang.Class, java.lang.Class[]) methodType}.

     * Any class or interface name embedded in the descriptor string will be

     * resolved by the given loader (or if it is null, on the system class loader).

     * <p>

     * Note that it is possible to encounter method types which cannot be

     * constructed by this method, because their component types are

     * not all reachable from a common class loader.

     * <p>

     * This method is included for the benefit of applications that must

     * generate bytecodes that process method handles and {@code invokedynamic}.

     * @param descriptor a bytecode-level type descriptor string "(T...)T"

     * @param loader the class loader in which to look up the types

     * @return a method type matching the bytecode-level type descriptor

     * @throws NullPointerException if the string is null

     * @throws IllegalArgumentException if the string is not well-formed

     * @throws TypeNotPresentException if a named type cannot be found

     * @throws SecurityException if the security manager is present and

     *         {@code loader} is {@code null} and the caller does not have the

     *         {@link RuntimePermission}{@code ("getClassLoader")}

     */

    public static MethodType fromMethodDescriptorString(String descriptor, ClassLoader loader)

        throws IllegalArgumentException, TypeNotPresentException

    {

        if (loader == null) {

            @SuppressWarnings("removal")

            SecurityManager sm = System.getSecurityManager();

            if (sm != null) {

                sm.checkPermission(SecurityConstants.GET_CLASSLOADER_PERMISSION);

            }

        }

        return fromDescriptor(descriptor,

                              (loader == null) ? ClassLoader.getSystemClassLoader() : loader);

    }

    /**

     * Same as {@link #fromMethodDescriptorString(String, ClassLoader)}, but

     * {@code null} ClassLoader means the bootstrap loader is used here.

     * <p>

     * IMPORTANT: This method is preferable for JDK internal use as it more

     * correctly interprets {@code null} ClassLoader than

     * {@link #fromMethodDescriptorString(String, ClassLoader)}.

     * Use of this method also avoids early initialization issues when system

     * ClassLoader is not initialized yet.

     */

    static MethodType fromDescriptor(String descriptor, ClassLoader loader)

        throws IllegalArgumentException, TypeNotPresentException

    {

        if (!descriptor.startsWith("(") ||  // also generates NPE if needed

            descriptor.indexOf(')') < 0 ||

            descriptor.indexOf('.') >= 0)

            throw newIllegalArgumentException("not a method descriptor: "+descriptor);

        List<Class<?>> types = BytecodeDescriptor.parseMethod(descriptor, loader);

        Class<?> rtype = types.remove(types.size() - 1);

        Class<?>[] ptypes = listToArray(types);

        return makeImpl(rtype, ptypes, true);

    }

    /**

     * Returns a descriptor string for the method type.  This method

     * is equivalent to calling {@link #descriptorString() MethodType::descriptorString}.

     *

     * <p>

     * Note that this is not a strict inverse of {@link #fromMethodDescriptorString fromMethodDescriptorString}.

     * Two distinct classes which share a common name but have different class loaders

     * will appear identical when viewed within descriptor strings.

     * <p>

     * This method is included for the benefit of applications that must

     * generate bytecodes that process method handles and {@code invokedynamic}.

     * {@link #fromMethodDescriptorString(java.lang.String, java.lang.ClassLoader) fromMethodDescriptorString},

     * because the latter requires a suitable class loader argument.

     * @return the descriptor string for this method type

     * @jvms 4.3.3 Method Descriptors

     * @see <a href="#descriptor">Nominal Descriptor for {@code MethodType}</a>

     */

    public String toMethodDescriptorString() {

        String desc = methodDescriptor;

        if (desc == null) {

            desc = BytecodeDescriptor.unparseMethod(this.rtype, this.ptypes);

            methodDescriptor = desc;

        }

        return desc;

    }

    /**

     * Returns a descriptor string for this method type.

     *

     * <p>

     * If this method type can be <a href="#descriptor">described nominally</a>,

     * then the result is a method type descriptor (JVMS {@jvms 4.3.3}).

     * {@link MethodTypeDesc MethodTypeDesc} for this method type

     * can be produced by calling {@link MethodTypeDesc#ofDescriptor(String)

     * MethodTypeDesc::ofDescriptor} with the result descriptor string.

     * <p>

     * If this method type cannot be <a href="#descriptor">described nominally</a>

     * and the result is a string of the form:

     * <blockquote>{@code "(<parameter-descriptors>)<return-descriptor>"}</blockquote>

     * where {@code <parameter-descriptors>} is the concatenation of the

     * {@linkplain Class#descriptorString() descriptor string} of all

     * of the parameter types and the {@linkplain Class#descriptorString() descriptor string}

     * of the return type. No {@link java.lang.constant.MethodTypeDesc MethodTypeDesc}

     * can be produced from the result string.

     *

     * @return the descriptor string for this method type

     * @since 12

     * @jvms 4.3.3 Method Descriptors

     * @see <a href="#descriptor">Nominal Descriptor for {@code MethodType}</a>

     */

    @Override

    public String descriptorString() {

        return toMethodDescriptorString();

    }

    /*non-public*/

    static String toFieldDescriptorString(Class<?> cls) {

        return BytecodeDescriptor.unparse(cls);

    }

    /**

     * Returns a nominal descriptor for this instance, if one can be

     * constructed, or an empty {@link Optional} if one cannot be.

     *

     * @return An {@link Optional} containing the resulting nominal descriptor,

     * or an empty {@link Optional} if one cannot be constructed.

     * @since 12

     * @see <a href="#descriptor">Nominal Descriptor for {@code MethodType}</a>

     */

    @Override

    public Optional<MethodTypeDesc> describeConstable() {

        try {

            return Optional.of(MethodTypeDesc.of(returnType().describeConstable().orElseThrow(),

                                                 Stream.of(parameterArray())

                                                      .map(p -> p.describeConstable().orElseThrow())

                                                      .toArray(ClassDesc[]::new)));

        }

        catch (NoSuchElementException e) {

            return Optional.empty();

        }

    }

    /// Serialization.

    /**

     * There are no serializable fields for {@code MethodType}.

     */

    @java.io.Serial

    private static final java.io.ObjectStreamField[] serialPersistentFields = { };

    /**

     * Save the {@code MethodType} instance to a stream.

     *

     * @serialData

     * For portability, the serialized format does not refer to named fields.

     * Instead, the return type and parameter type arrays are written directly