/*
 * Copyright (c) 2012, 2020, 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
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package com.sun.tools.javac.comp;

import com.sun.source.tree.LambdaExpressionTree.BodyKind;
import com.sun.source.tree.NewClassTree;
import com.sun.tools.javac.code.*;
import com.sun.tools.javac.code.Type.ErrorType;
import com.sun.tools.javac.code.Type.MethodType;
import com.sun.tools.javac.code.Type.StructuralTypeMapping;
import com.sun.tools.javac.code.Types.TypeMapping;
import com.sun.tools.javac.comp.ArgumentAttr.LocalCacheContext;
import com.sun.tools.javac.comp.Infer.GraphSolver.InferenceGraph;
import com.sun.tools.javac.comp.Resolve.ResolveError;
import com.sun.tools.javac.resources.CompilerProperties.Fragments;
import com.sun.tools.javac.tree.*;
import com.sun.tools.javac.util.*;
import com.sun.tools.javac.util.DefinedBy.Api;
import com.sun.tools.javac.util.GraphUtils.DependencyKind;
import com.sun.tools.javac.util.JCDiagnostic.DiagnosticPosition;
import com.sun.tools.javac.comp.Attr.ResultInfo;
import com.sun.tools.javac.comp.Resolve.MethodResolutionPhase;
import com.sun.tools.javac.resources.CompilerProperties.Errors;
import com.sun.tools.javac.tree.JCTree.*;
import com.sun.tools.javac.util.JCDiagnostic.DiagnosticType;
import com.sun.tools.javac.util.Log.DeferredDiagnosticHandler;
import com.sun.tools.javac.util.Log.DiagnosticHandler;

import java.util.ArrayList;
import java.util.Collection;
import java.util.Collections;
import java.util.EnumSet;
import java.util.HashSet;
import java.util.LinkedHashSet;
import java.util.Map;
import java.util.Set;
import java.util.WeakHashMap;
import java.util.function.Supplier;

import com.sun.source.tree.MemberReferenceTree;
import com.sun.tools.javac.code.Type;
import com.sun.tools.javac.tree.JCTree.JCMemberReference.OverloadKind;

import static com.sun.tools.javac.code.TypeTag.*;
import com.sun.tools.javac.comp.Annotate.Queues;
import static com.sun.tools.javac.tree.JCTree.Tag.*;

This is an helper class that is used to perform deferred type-analysis.
Each time a poly expression occurs in argument position, javac attributes it
with a temporary 'deferred type' that is checked (possibly multiple times)
against an expected formal type.
 
This is NOT part of any supported API.
 If you write code that depends on this, you do so at your own risk.
 This code and its internal interfaces are subject to change or
 deletion without notice.
/**
 * This is an helper class that is used to perform deferred type-analysis.
 * Each time a poly expression occurs in argument position, javac attributes it
 * with a temporary 'deferred type' that is checked (possibly multiple times)
 * against an expected formal type.
 *
 *  <p><b>This is NOT part of any supported API.
 *  If you write code that depends on this, you do so at your own risk.
 *  This code and its internal interfaces are subject to change or
 *  deletion without notice.</b>
 */
public class DeferredAttr extends JCTree.Visitor {
    protected static final Context.Key<DeferredAttr> deferredAttrKey = new Context.Key<>();

    final Annotate annotate;
    final Attr attr;
    final ArgumentAttr argumentAttr;
    final Check chk;
    final JCDiagnostic.Factory diags;
    final Enter enter;
    final Infer infer;
    final Resolve rs;
    final Log log;
    final Symtab syms;
    final TreeMaker make;
    final TreeCopier<Void> treeCopier;
    final TypeMapping<Void> deferredCopier;
    final Types types;
    final Flow flow;
    final Names names;
    final TypeEnvs typeEnvs;
    final DeferredCompletionFailureHandler dcfh;

    public static DeferredAttr instance(Context context) {
        DeferredAttr instance = context.get(deferredAttrKey);
        if (instance == null)
            instance = new DeferredAttr(context);
        return instance;
    }

    protected DeferredAttr(Context context) {
        context.put(deferredAttrKey, this);
        annotate = Annotate.instance(context);
        attr = Attr.instance(context);
        argumentAttr = ArgumentAttr.instance(context);
        chk = Check.instance(context);
        diags = JCDiagnostic.Factory.instance(context);
        enter = Enter.instance(context);
        infer = Infer.instance(context);
        rs = Resolve.instance(context);
        log = Log.instance(context);
        syms = Symtab.instance(context);
        make = TreeMaker.instance(context);
        types = Types.instance(context);
        flow = Flow.instance(context);
        names = Names.instance(context);
        stuckTree = make.Ident(names.empty).setType(Type.stuckType);
        typeEnvs = TypeEnvs.instance(context);
        dcfh = DeferredCompletionFailureHandler.instance(context);
        emptyDeferredAttrContext =
            new DeferredAttrContext(AttrMode.CHECK, null, MethodResolutionPhase.BOX, infer.emptyContext, null, null) {
                @Override
                void addDeferredAttrNode(DeferredType dt, ResultInfo ri, DeferredStuckPolicy deferredStuckPolicy) {
                    Assert.error("Empty deferred context!");
                }
                @Override
                void complete() {
                    Assert.error("Empty deferred context!");
                }

                @Override
                public String toString() {
                    return "Empty deferred context!";
                }
            };

        // For speculative attribution, skip the class definition in <>.
        treeCopier =
            new TreeCopier<Void>(make) {
                @Override @DefinedBy(Api.COMPILER_TREE)
                public JCTree visitNewClass(NewClassTree node, Void p) {
                    JCNewClass t = (JCNewClass) node;
                    if (TreeInfo.isDiamond(t)) {
                        JCExpression encl = copy(t.encl, p);
                        List<JCExpression> typeargs = copy(t.typeargs, p);
                        JCExpression clazz = copy(t.clazz, p);
                        List<JCExpression> args = copy(t.args, p);
                        JCClassDecl def = null;
                        return make.at(t.pos).SpeculativeNewClass(encl, typeargs, clazz, args, def, t.def != null);
                    } else {
                        return super.visitNewClass(node, p);
                    }
                }

                @Override @DefinedBy(Api.COMPILER_TREE)
                public JCTree visitMemberReference(MemberReferenceTree node, Void p) {
                    JCMemberReference t = (JCMemberReference) node;
                    JCExpression expr = copy(t.expr, p);
                    List<JCExpression> typeargs = copy(t.typeargs, p);
                    /** once the value for overloadKind is determined for a copy, it can be safely forwarded to
                     *  the copied tree, we want to profit from that
                     */
                    JCMemberReference result = new JCMemberReference(t.mode, t.name, expr, typeargs) {
                        @Override
                        public void setOverloadKind(OverloadKind overloadKind) {
                            OverloadKind previous = t.getOverloadKind();
                            if (previous == null || previous == OverloadKind.ERROR) {
                                t.setOverloadKind(overloadKind);
                            } else {
                                Assert.check(previous == overloadKind || overloadKind == OverloadKind.ERROR);
                            }
                        }

                        @Override
                        public OverloadKind getOverloadKind() {
                            return t.getOverloadKind();
                        }
                    };
                    result.pos = t.pos;
                    return result;
                }
            };
        deferredCopier = new TypeMapping<Void> () {
                @Override
                public Type visitType(Type t, Void v) {
                    if (t.hasTag(DEFERRED)) {
                        DeferredType dt = (DeferredType) t;
                        return new DeferredType(treeCopier.copy(dt.tree), dt.env);
                    }
                    return t;
                }
            };
    }

    
shared tree for stuck expressions /** shared tree for stuck expressions */
    final JCTree stuckTree;

    
This type represents a deferred type. A deferred type starts off with
no information on the underlying expression type. Such info needs to be
discovered through type-checking the deferred type against a target-type.
Every deferred type keeps a pointer to the AST node from which it originated.
/**
     * This type represents a deferred type. A deferred type starts off with
     * no information on the underlying expression type. Such info needs to be
     * discovered through type-checking the deferred type against a target-type.
     * Every deferred type keeps a pointer to the AST node from which it originated.
     */
    public class DeferredType extends Type {

        public JCExpression tree;
        Env<AttrContext> env;
        AttrMode mode;
        Set<Symbol> notPertinentToApplicability = new HashSet<>();
        SpeculativeCache speculativeCache;

        DeferredType(JCExpression tree, Env<AttrContext> env) {
            super(null, TypeMetadata.EMPTY);
            this.tree = tree;
            this.env = attr.copyEnv(env);
            this.speculativeCache = new SpeculativeCache();
        }

        @Override
        public DeferredType cloneWithMetadata(TypeMetadata md) {
            throw new AssertionError("Cannot add metadata to a deferred type");
        }

        @Override
        public TypeTag getTag() {
            return DEFERRED;
        }

        @Override @DefinedBy(Api.LANGUAGE_MODEL)
        public String toString() {
            return "DeferredType";
        }

        
A speculative cache is used to keep track of all overload resolution rounds
that triggered speculative attribution on a given deferred type. Each entry
stores a pointer to the speculative tree and the resolution phase in which the entry
has been added.
/**
         * A speculative cache is used to keep track of all overload resolution rounds
         * that triggered speculative attribution on a given deferred type. Each entry
         * stores a pointer to the speculative tree and the resolution phase in which the entry
         * has been added.
         */
        class SpeculativeCache {

            private Map<Symbol, List<Entry>> cache = new WeakHashMap<>();

            class Entry {
                JCTree speculativeTree;
                ResultInfo resultInfo;

                public Entry(JCTree speculativeTree, ResultInfo resultInfo) {
                    this.speculativeTree = speculativeTree;
                    this.resultInfo = resultInfo;
                }

                boolean matches(MethodResolutionPhase phase) {
                    return resultInfo.checkContext.deferredAttrContext().phase == phase;
                }
            }

            
Retrieve a speculative cache entry corresponding to given symbol
and resolution phase
/**
             * Retrieve a speculative cache entry corresponding to given symbol
             * and resolution phase
             */
            Entry get(Symbol msym, MethodResolutionPhase phase) {
                List<Entry> entries = cache.get(msym);
                if (entries == null) return null;
                for (Entry e : entries) {
                    if (e.matches(phase)) return e;
                }
                return null;
            }

            
Stores a speculative cache entry corresponding to given symbol
and resolution phase
/**
             * Stores a speculative cache entry corresponding to given symbol
             * and resolution phase
             */
            void put(JCTree speculativeTree, ResultInfo resultInfo) {
                Symbol msym = resultInfo.checkContext.deferredAttrContext().msym;
                List<Entry> entries = cache.get(msym);
                if (entries == null) {
                    entries = List.nil();
                }
                cache.put(msym, entries.prepend(new Entry(speculativeTree, resultInfo)));
            }
        }

        
Get the type that has been computed during a speculative attribution round
/**
         * Get the type that has been computed during a speculative attribution round
         */
        Type speculativeType(Symbol msym, MethodResolutionPhase phase) {
            SpeculativeCache.Entry e = speculativeCache.get(msym, phase);
            return e != null ? e.speculativeTree.type : Type.noType;
        }

        JCTree speculativeTree(DeferredAttrContext deferredAttrContext) {
            DeferredType.SpeculativeCache.Entry e = speculativeCache.get(deferredAttrContext.msym, deferredAttrContext.phase);
            return e != null ? e.speculativeTree : stuckTree;
        }

        DeferredTypeCompleter completer() {
            return basicCompleter;
        }

        
Check a deferred type against a potential target-type. Depending on
the current attribution mode, a normal vs. speculative attribution
round is performed on the underlying AST node. There can be only one
speculative round for a given target method symbol; moreover, a normal
attribution round must follow one or more speculative rounds.
/**
         * Check a deferred type against a potential target-type. Depending on
         * the current attribution mode, a normal vs. speculative attribution
         * round is performed on the underlying AST node. There can be only one
         * speculative round for a given target method symbol; moreover, a normal
         * attribution round must follow one or more speculative rounds.
         */
        Type check(ResultInfo resultInfo) {
            DeferredStuckPolicy deferredStuckPolicy;
            if (resultInfo.pt.hasTag(NONE) || resultInfo.pt.isErroneous()) {
                deferredStuckPolicy = dummyStuckPolicy;
            } else if (resultInfo.checkContext.deferredAttrContext().mode == AttrMode.SPECULATIVE ||
                    resultInfo.checkContext.deferredAttrContext().insideOverloadPhase()) {
                deferredStuckPolicy = new OverloadStuckPolicy(resultInfo, this);
            } else {
                deferredStuckPolicy = new CheckStuckPolicy(resultInfo, this);
            }
            return check(resultInfo, deferredStuckPolicy, completer());
        }

        private Type check(ResultInfo resultInfo, DeferredStuckPolicy deferredStuckPolicy,
                DeferredTypeCompleter deferredTypeCompleter) {
            DeferredAttrContext deferredAttrContext =
                    resultInfo.checkContext.deferredAttrContext();
            Assert.check(deferredAttrContext != emptyDeferredAttrContext);
            if (deferredStuckPolicy.isStuck()) {
                notPertinentToApplicability.add(deferredAttrContext.msym);
                deferredAttrContext.addDeferredAttrNode(this, resultInfo, deferredStuckPolicy);
                return Type.noType;
            } else {
                try {
                    return deferredTypeCompleter.complete(this, resultInfo, deferredAttrContext);
                } finally {
                    mode = deferredAttrContext.mode;
                }
            }
        }
    }

    
A completer for deferred types. Defines an entry point for type-checking
a deferred type.
/**
     * A completer for deferred types. Defines an entry point for type-checking
     * a deferred type.
     */
    interface DeferredTypeCompleter {
        
Entry point for type-checking a deferred type. Depending on the
circumstances, type-checking could amount to full attribution
or partial structural check (aka potential applicability).
/**
         * Entry point for type-checking a deferred type. Depending on the
         * circumstances, type-checking could amount to full attribution
         * or partial structural check (aka potential applicability).
         */
        Type complete(DeferredType dt, ResultInfo resultInfo, DeferredAttrContext deferredAttrContext);
    }


    
A basic completer for deferred types. This completer type-checks a deferred type
using attribution; depending on the attribution mode, this could be either standard
or speculative attribution.
/**
     * A basic completer for deferred types. This completer type-checks a deferred type
     * using attribution; depending on the attribution mode, this could be either standard
     * or speculative attribution.
     */
    DeferredTypeCompleter basicCompleter = new DeferredTypeCompleter() {
        public Type complete(DeferredType dt, ResultInfo resultInfo, DeferredAttrContext deferredAttrContext) {
            switch (deferredAttrContext.mode) {
                case SPECULATIVE:
                    //Note: if a symbol is imported twice we might do two identical
                    //speculative rounds...
                    Assert.check(dt.mode == null || dt.mode == AttrMode.SPECULATIVE);
                    JCTree speculativeTree = attribSpeculative(dt.tree, dt.env, resultInfo);
                    dt.speculativeCache.put(speculativeTree, resultInfo);
                    return speculativeTree.type;
                case CHECK:
                    Assert.check(dt.mode != null);
                    return attr.attribTree(dt.tree, dt.env, resultInfo);
            }
            Assert.error();
            return null;
        }
    };

    
Policy for detecting stuck expressions. Different criteria might cause
an expression to be judged as stuck, depending on whether the check
is performed during overload resolution or after most specific.
/**
     * Policy for detecting stuck expressions. Different criteria might cause
     * an expression to be judged as stuck, depending on whether the check
     * is performed during overload resolution or after most specific.
     */
    interface DeferredStuckPolicy {
        
Has the policy detected that a given expression should be considered stuck?
/**
         * Has the policy detected that a given expression should be considered stuck?
         */
        boolean isStuck();
        
Get the set of inference variables a given expression depends upon.
/**
         * Get the set of inference variables a given expression depends upon.
         */
        Set<Type> stuckVars();
        
Get the set of inference variables which might get new constraints
if a given expression is being type-checked.
/**
         * Get the set of inference variables which might get new constraints
         * if a given expression is being type-checked.
         */
        Set<Type> depVars();
    }

    
Basic stuck policy; an expression is never considered to be stuck.
/**
     * Basic stuck policy; an expression is never considered to be stuck.
     */
    DeferredStuckPolicy dummyStuckPolicy = new DeferredStuckPolicy() {
        @Override
        public boolean isStuck() {
            return false;
        }
        @Override
        public Set<Type> stuckVars() {
            return Collections.emptySet();
        }
        @Override
        public Set<Type> depVars() {
            return Collections.emptySet();
        }
    };

    
The 'mode' in which the deferred type is to be type-checked
/**
     * The 'mode' in which the deferred type is to be type-checked
     */
    public enum AttrMode {
        
A speculative type-checking round is used during overload resolution
mainly to generate constraints on inference variables. Side-effects
arising from type-checking the expression associated with the deferred
type are reversed after the speculative round finishes. This means the
expression tree will be left in a blank state.
/**
         * A speculative type-checking round is used during overload resolution
         * mainly to generate constraints on inference variables. Side-effects
         * arising from type-checking the expression associated with the deferred
         * type are reversed after the speculative round finishes. This means the
         * expression tree will be left in a blank state.
         */
        SPECULATIVE,
        
This is the plain type-checking mode. Produces side-effects on the underlying AST node
/**
         * This is the plain type-checking mode. Produces side-effects on the underlying AST node
         */
        CHECK
    }

    
Performs speculative attribution of a lambda body and returns the speculative lambda tree, in the absence of a target-type. Since Attr.visitLambda(JCLambda) cannot type-check lambda bodies w/o a suitable target-type, this routine 'unrolls' the lambda by turning it into a regular block, speculatively type-checks the block and then puts back the pieces. /**
     * Performs speculative attribution of a lambda body and returns the speculative lambda tree,
     * in the absence of a target-type. Since {@link Attr#visitLambda(JCLambda)} cannot type-check
     * lambda bodies w/o a suitable target-type, this routine 'unrolls' the lambda by turning it
     * into a regular block, speculatively type-checks the block and then puts back the pieces.
     */
    JCLambda attribSpeculativeLambda(JCLambda that, Env<AttrContext> env, ResultInfo resultInfo) {
        ListBuffer<JCStatement> stats = new ListBuffer<>();
        stats.addAll(that.params);
        if (that.getBodyKind() == JCLambda.BodyKind.EXPRESSION) {
            stats.add(make.Return((JCExpression)that.body));
        } else {
            stats.add((JCBlock)that.body);
        }
        JCBlock lambdaBlock = make.at(that.pos).Block(0, stats.toList());
        Env<AttrContext> localEnv = attr.lambdaEnv(that, env);
        try {
            localEnv.info.returnResult = resultInfo;
            JCBlock speculativeTree = (JCBlock)attribSpeculative(lambdaBlock, localEnv, resultInfo);
            List<JCVariableDecl> args = speculativeTree.getStatements().stream()
                    .filter(s -> s.hasTag(Tag.VARDEF))
                    .map(t -> (JCVariableDecl)t)
                    .collect(List.collector());
            JCTree lambdaBody = speculativeTree.getStatements().last();
            if (lambdaBody.hasTag(Tag.RETURN)) {
                lambdaBody = ((JCReturn)lambdaBody).expr;
            }
            JCLambda speculativeLambda = make.Lambda(args, lambdaBody);
            attr.preFlow(speculativeLambda);
            flow.analyzeLambda(env, speculativeLambda, make, false);
            return speculativeLambda;
        } finally {
            localEnv.info.scope.leave();
        }
    }

    
Routine that performs speculative type-checking; the input AST node is
cloned (to avoid side-effects cause by Attr) and compiler state is
restored after type-checking. All diagnostics (but critical ones) are
disabled during speculative type-checking.
/**
     * Routine that performs speculative type-checking; the input AST node is
     * cloned (to avoid side-effects cause by Attr) and compiler state is
     * restored after type-checking. All diagnostics (but critical ones) are
     * disabled during speculative type-checking.
     */
    JCTree attribSpeculative(JCTree tree, Env<AttrContext> env, ResultInfo resultInfo) {
        return attribSpeculative(tree, env, resultInfo, treeCopier,
                null, AttributionMode.SPECULATIVE, null);
    }

    JCTree attribSpeculative(JCTree tree, Env<AttrContext> env, ResultInfo resultInfo, LocalCacheContext localCache) {
        return attribSpeculative(tree, env, resultInfo, treeCopier,
                null, AttributionMode.SPECULATIVE, localCache);
    }

    <Z> JCTree attribSpeculative(JCTree tree, Env<AttrContext> env, ResultInfo resultInfo, TreeCopier<Z> deferredCopier,
                                 Supplier<DiagnosticHandler> diagHandlerCreator, AttributionMode attributionMode,
                                 LocalCacheContext localCache) {
        final JCTree newTree = deferredCopier.copy(tree);
        return attribSpeculative(newTree, env, resultInfo, diagHandlerCreator, attributionMode, localCache);
    }

    
Attribute the given tree, mostly reverting side-effects applied to shared
compiler state. Exceptions include the ArgumentAttr.argumentTypeCache,
changes to which may be preserved if localCache is null and errors reported
outside of the speculatively attributed tree.
/**
     * Attribute the given tree, mostly reverting side-effects applied to shared
     * compiler state. Exceptions include the ArgumentAttr.argumentTypeCache,
     * changes to which may be preserved if localCache is null and errors reported
     * outside of the speculatively attributed tree.
     */
    <Z> JCTree attribSpeculative(JCTree tree, Env<AttrContext> env, ResultInfo resultInfo,
                              Supplier<DiagnosticHandler> diagHandlerCreator, AttributionMode attributionMode,
                              LocalCacheContext localCache) {
        Env<AttrContext> speculativeEnv = env.dup(tree, env.info.dup(env.info.scope.dupUnshared(env.info.scope.owner)));
        speculativeEnv.info.attributionMode = attributionMode;
        Log.DiagnosticHandler deferredDiagnosticHandler = diagHandlerCreator != null ? diagHandlerCreator.get() : new DeferredAttrDiagHandler(log, tree);
        DeferredCompletionFailureHandler.Handler prevCFHandler = dcfh.setHandler(dcfh.speculativeCodeHandler);
        Queues prevQueues = annotate.setQueues(new Queues());
        int nwarnings = log.nwarnings;
        log.nwarnings = 0;
        try {
            attr.attribTree(tree, speculativeEnv, resultInfo);
            return tree;
        } finally {
            annotate.setQueues(prevQueues);
            dcfh.setHandler(prevCFHandler);
            log.nwarnings += nwarnings;
            enter.unenter(env.toplevel, tree);
            log.popDiagnosticHandler(deferredDiagnosticHandler);
            if (localCache != null) {
                localCache.leave();
            }
        }
    }
    //where
        static class DeferredAttrDiagHandler extends Log.DeferredDiagnosticHandler {

            static class PosScanner extends TreeScanner {
                DiagnosticPosition pos;
                boolean found = false;

                PosScanner(DiagnosticPosition pos) {
                    this.pos = pos;
                }

                @Override
                public void scan(JCTree tree) {
                    if (tree != null &&
                            tree.pos() == pos) {
                        found = true;
                    }
                    super.scan(tree);
                }
            }

            DeferredAttrDiagHandler(Log log, JCTree newTree) {
                super(log, d -> {
                    PosScanner posScanner = new PosScanner(d.getDiagnosticPosition());
                    posScanner.scan(newTree);
                    return posScanner.found;
                });
            }
        }

    
A deferred context is created on each method check. A deferred context is
used to keep track of information associated with the method check, such as
the symbol of the method being checked, the overload resolution phase,
the kind of attribution mode to be applied to deferred types and so forth.
As deferred types are processed (by the method check routine) stuck AST nodes
are added (as new deferred attribution nodes) to this context. The complete()
routine makes sure that all pending nodes are properly processed, by
progressively instantiating all inference variables on which one or more
deferred attribution node is stuck.
/**
     * A deferred context is created on each method check. A deferred context is
     * used to keep track of information associated with the method check, such as
     * the symbol of the method being checked, the overload resolution phase,
     * the kind of attribution mode to be applied to deferred types and so forth.
     * As deferred types are processed (by the method check routine) stuck AST nodes
     * are added (as new deferred attribution nodes) to this context. The complete()
     * routine makes sure that all pending nodes are properly processed, by
     * progressively instantiating all inference variables on which one or more
     * deferred attribution node is stuck.
     */
    class DeferredAttrContext {

        
attribution mode /** attribution mode */
        final AttrMode mode;

        
symbol of the method being checked /** symbol of the method being checked */
        final Symbol msym;

        
method resolution step /** method resolution step */
        final Resolve.MethodResolutionPhase phase;

        
inference context /** inference context */
        final InferenceContext inferenceContext;

        
parent deferred context /** parent deferred context */
        final DeferredAttrContext parent;

        
Warner object to report warnings /** Warner object to report warnings */
        final Warner warn;

        
list of deferred attribution nodes to be processed /** list of deferred attribution nodes to be processed */
        ArrayList<DeferredAttrNode> deferredAttrNodes = new ArrayList<>();

        DeferredAttrContext(AttrMode mode, Symbol msym, MethodResolutionPhase phase,
                InferenceContext inferenceContext, DeferredAttrContext parent, Warner warn) {
            this.mode = mode;
            this.msym = msym;
            this.phase = phase;
            this.parent = parent;
            this.warn = warn;
            this.inferenceContext = inferenceContext;
        }

        
Adds a node to the list of deferred attribution nodes - used by Resolve.rawCheckArgumentsApplicable
Nodes added this way act as 'roots' for the out-of-order method checking process.
/**
         * Adds a node to the list of deferred attribution nodes - used by Resolve.rawCheckArgumentsApplicable
         * Nodes added this way act as 'roots' for the out-of-order method checking process.
         */
        void addDeferredAttrNode(final DeferredType dt, ResultInfo resultInfo,
                DeferredStuckPolicy deferredStuckPolicy) {
            deferredAttrNodes.add(new DeferredAttrNode(dt, resultInfo, deferredStuckPolicy));
        }

        
Incrementally process all nodes, by skipping 'stuck' nodes and attributing
'unstuck' ones. If at any point no progress can be made (no 'unstuck' nodes)
some inference variable might get eagerly instantiated so that all nodes
can be type-checked.
/**
         * Incrementally process all nodes, by skipping 'stuck' nodes and attributing
         * 'unstuck' ones. If at any point no progress can be made (no 'unstuck' nodes)
         * some inference variable might get eagerly instantiated so that all nodes
         * can be type-checked.
         */
        void complete() {
            while (!deferredAttrNodes.isEmpty()) {
                boolean progress = false;
                //scan a defensive copy of the node list - this is because a deferred
                //attribution round can add new nodes to the list
                for (DeferredAttrNode deferredAttrNode : List.from(deferredAttrNodes)) {
                    if (deferredAttrNode.process(this)) {
                        deferredAttrNodes.remove(deferredAttrNode);
                        progress = true;
                    }
                }
                if (!progress) {
                    if (insideOverloadPhase()) {
                        for (DeferredAttrNode deferredNode: deferredAttrNodes) {
                            deferredNode.dt.tree.type = Type.noType;
                        }
                        return;
                    }
                    //remove all variables that have already been instantiated
                    //from the list of stuck variables
                    try {
                        //find stuck expression to unstuck
                        DeferredAttrNode toUnstuck = pickDeferredNode();
                        inferenceContext.solveAny(List.from(toUnstuck.deferredStuckPolicy.stuckVars()), warn);
                        inferenceContext.notifyChange();
                    } catch (Infer.GraphStrategy.NodeNotFoundException ex) {
                        //this means that we are in speculative mode and the
                        //set of constraints are too tight for progress to be made.
                        //Just leave the remaining expressions as stuck.
                        break;
                    }
                }
            }
        }

        public boolean insideOverloadPhase() {
            DeferredAttrContext dac = this;
            if (dac == emptyDeferredAttrContext) {
                return false;
            }
            if (dac.mode == AttrMode.SPECULATIVE) {
                return true;
            }
            return dac.parent.insideOverloadPhase();
        }

        
Pick the deferred node to be unstuck. First, deferred nodes are organized into a graph (see DeferredAttrContext.buildStuckGraph(), where a node N1 depends on another node N2 if its input variable depends (as per the inference graph) on the output variables of N2 (see DeferredAttrContext.canInfluence(). Then, the chosen deferred node is the first strongly connected component containing exactly one node found in such a graph. If no such component is found, the first deferred node is chosen. /**
         * Pick the deferred node to be unstuck. First, deferred nodes are organized into a graph
         * (see {@code DeferredAttrContext.buildStuckGraph()}, where a node N1 depends on another node N2
         * if its input variable depends (as per the inference graph) on the output variables of N2
         * (see {@code DeferredAttrContext.canInfluence()}.
         *
         * Then, the chosen deferred node is the first strongly connected component containing exactly
         * one node found in such a graph. If no such component is found, the first deferred node is chosen.
         */
        DeferredAttrNode pickDeferredNode() {
            List<StuckNode> stuckGraph = buildStuckGraph();
            //compute tarjan on the stuck graph
            List<? extends StuckNode> csn = GraphUtils.tarjan(stuckGraph).get(0);
            return csn.length() == 1 ? csn.get(0).data : deferredAttrNodes.get(0);
        }

        List<StuckNode> buildStuckGraph() {
            //first, build inference graph
            infer.doIncorporation(inferenceContext, warn);
            InferenceGraph graph = infer.new GraphSolver(inferenceContext, types.noWarnings)
                    .new InferenceGraph();
            //then, build stuck graph
            List<StuckNode> nodes = deferredAttrNodes.stream()
                    .map(StuckNode::new)
                    .collect(List.collector());
            //init stuck expression graph; a deferred node A depends on a deferred node B iff
            //B's output variables can influence A's input variables.
            for (StuckNode sn1 : nodes) {
                for (StuckNode sn2 : nodes) {
                    if (sn1 != sn2 && canInfluence(graph, sn2, sn1)) {
                        sn1.deps.add(sn2);
                    }
                }
            }
            return nodes;
        }

        boolean canInfluence(InferenceGraph graph, StuckNode sn1, StuckNode sn2) {
            Set<Type> outputVars = sn1.data.deferredStuckPolicy.depVars();
            for (Type inputVar : sn2.data.deferredStuckPolicy.stuckVars()) {
                InferenceGraph.Node inputNode = graph.findNode(inputVar);
                //already solved stuck vars do not appear in the graph
                if (inputNode != null) {
                    Set<InferenceGraph.Node> inputClosure = inputNode.closure();
                    if (outputVars.stream()
                            .map(graph::findNode)
                            .anyMatch(inputClosure::contains)) {
                        return true;
                    }
                }
            }
            return false;
        }

        class StuckNode extends GraphUtils.TarjanNode<DeferredAttrNode, StuckNode> {

            Set<StuckNode> deps = new HashSet<>();

            StuckNode(DeferredAttrNode data) {
                super(data);
            }

            @Override
            public DependencyKind[] getSupportedDependencyKinds() {
                return new DependencyKind[] { Infer.DependencyKind.STUCK };
            }

            @Override
            public Collection<? extends StuckNode> getDependenciesByKind(DependencyKind dk) {
                if (dk == Infer.DependencyKind.STUCK) {
                    return deps;
                } else {
                    throw new IllegalStateException();
                }
            }

            @Override
            public Iterable<? extends StuckNode> getAllDependencies() {
                return deps;
            }
        }
    }

    
Class representing a deferred attribution node. It keeps track of
a deferred type, along with the expected target type information.
/**
     * Class representing a deferred attribution node. It keeps track of
     * a deferred type, along with the expected target type information.
     */
    class DeferredAttrNode {

        
underlying deferred type /** underlying deferred type */
        DeferredType dt;

        
underlying target type information /** underlying target type information */
        ResultInfo resultInfo;

        
stuck policy associated with this node /** stuck policy associated with this node */
        DeferredStuckPolicy deferredStuckPolicy;

        DeferredAttrNode(DeferredType dt, ResultInfo resultInfo, DeferredStuckPolicy deferredStuckPolicy) {
            this.dt = dt;
            this.resultInfo = resultInfo;
            this.deferredStuckPolicy = deferredStuckPolicy;
        }

        
Process a deferred attribution node.
Invariant: a stuck node cannot be processed.
/**
         * Process a deferred attribution node.
         * Invariant: a stuck node cannot be processed.
         */
        @SuppressWarnings("fallthrough")
        boolean process(final DeferredAttrContext deferredAttrContext) {
            switch (deferredAttrContext.mode) {
                case SPECULATIVE:
                    if (deferredStuckPolicy.isStuck()) {
                        dt.check(resultInfo, dummyStuckPolicy, new StructuralStuckChecker());
                        return true;
                    } else {
                        Assert.error("Cannot get here");
                    }
                case CHECK:
                    if (deferredStuckPolicy.isStuck()) {
                        //stuck expression - see if we can propagate
                        if (deferredAttrContext.parent != emptyDeferredAttrContext &&
                                Type.containsAny(deferredAttrContext.parent.inferenceContext.inferencevars,
                                        List.from(deferredStuckPolicy.stuckVars()))) {
                            deferredAttrContext.parent.addDeferredAttrNode(dt,
                                    resultInfo.dup(new Check.NestedCheckContext(resultInfo.checkContext) {
                                @Override
                                public InferenceContext inferenceContext() {
                                    return deferredAttrContext.parent.inferenceContext;
                                }
                                @Override
                                public DeferredAttrContext deferredAttrContext() {
                                    return deferredAttrContext.parent;
                                }
                            }), deferredStuckPolicy);
                            dt.tree.type = Type.stuckType;
                            return true;
                        } else {
                            return false;
                        }
                    } else {
                        Assert.check(!deferredAttrContext.insideOverloadPhase(),
                                "attribution shouldn't be happening here");
                        ResultInfo instResultInfo =
                                resultInfo.dup(deferredAttrContext.inferenceContext.asInstType(resultInfo.pt));
                        dt.check(instResultInfo, dummyStuckPolicy, basicCompleter);
                        return true;
                    }
                default:
                    throw new AssertionError("Bad mode");
            }
        }

        
Structural checker for stuck expressions
/**
         * Structural checker for stuck expressions
         */
        class StructuralStuckChecker extends TreeScanner implements DeferredTypeCompleter {

            ResultInfo resultInfo;
            InferenceContext inferenceContext;
            Env<AttrContext> env;

            public Type complete(DeferredType dt, ResultInfo resultInfo, DeferredAttrContext deferredAttrContext) {
                this.resultInfo = resultInfo;
                this.inferenceContext = deferredAttrContext.inferenceContext;
                this.env = dt.env;
                dt.tree.accept(this);
                dt.speculativeCache.put(stuckTree, resultInfo);
                return Type.noType;
            }

            @Override
            public void visitLambda(JCLambda tree) {
                Check.CheckContext checkContext = resultInfo.checkContext;
                Type pt = resultInfo.pt;
                if (!inferenceContext.inferencevars.contains(pt)) {
                    //must be a functional descriptor
                    Type descriptorType = null;
                    try {
                        descriptorType = types.findDescriptorType(pt);
                    } catch (Types.FunctionDescriptorLookupError ex) {
                        checkContext.report(null, ex.getDiagnostic());
                    }

                    if (descriptorType.getParameterTypes().length() != tree.params.length()) {
                        checkContext.report(tree,
                                diags.fragment(Fragments.IncompatibleArgTypesInLambda));
                    }

                    Type currentReturnType = descriptorType.getReturnType();
                    boolean returnTypeIsVoid = currentReturnType.hasTag(VOID);
                    if (tree.getBodyKind() == BodyKind.EXPRESSION) {
                        boolean isExpressionCompatible = !returnTypeIsVoid ||
                            TreeInfo.isExpressionStatement((JCExpression)tree.getBody());
                        if (!isExpressionCompatible) {
                            resultInfo.checkContext.report(tree.pos(),
                                diags.fragment(Fragments.IncompatibleRetTypeInLambda(Fragments.MissingRetVal(currentReturnType))));
                        }
                    } else {
                        LambdaBodyStructChecker lambdaBodyChecker =
                                new LambdaBodyStructChecker();

                        tree.body.accept(lambdaBodyChecker);
                        boolean isVoidCompatible = lambdaBodyChecker.isVoidCompatible;

                        if (returnTypeIsVoid) {
                            if (!isVoidCompatible) {
                                resultInfo.checkContext.report(tree.pos(),
                                    diags.fragment(Fragments.UnexpectedRetVal));
                            }
                        } else {
                            boolean isValueCompatible = lambdaBodyChecker.isPotentiallyValueCompatible
                                && !canLambdaBodyCompleteNormally(tree);
                            if (!isValueCompatible && !isVoidCompatible) {
                                log.error(tree.body.pos(),
                                          Errors.LambdaBodyNeitherValueNorVoidCompatible);
                            }

                            if (!isValueCompatible) {
                                resultInfo.checkContext.report(tree.pos(),
                                    diags.fragment(Fragments.IncompatibleRetTypeInLambda(Fragments.MissingRetVal(currentReturnType))));
                            }
                        }
                    }
                }
            }

            boolean canLambdaBodyCompleteNormally(JCLambda tree) {
                List<JCVariableDecl> oldParams = tree.params;
                LocalCacheContext localCacheContext = argumentAttr.withLocalCacheContext();
                try {
                    tree.params = tree.params.stream()
                            .map(vd -> make.VarDef(vd.mods, vd.name, make.Erroneous(), null))
                            .collect(List.collector());
                    return attribSpeculativeLambda(tree, env, attr.unknownExprInfo).canCompleteNormally;
                } finally {
                    localCacheContext.leave();
                    tree.params = oldParams;
                }
            }

            @Override
            public void visitNewClass(JCNewClass tree) {
                //do nothing
            }

            @Override
            public void visitApply(JCMethodInvocation tree) {
                //do nothing
            }

            @Override
            public void visitConditional(JCTree.JCConditional tree) {
                //skip tree.cond
                scan(tree.truepart);
                scan(tree.falsepart);
            }

            @Override
            public void visitReference(JCMemberReference tree) {
                Assert.checkNonNull(tree.getOverloadKind());
                Check.CheckContext checkContext = resultInfo.checkContext;
                Type pt = resultInfo.pt;
                if (!inferenceContext.inferencevars.contains(pt)) {
                    Type descriptor = null;
                    try {
                        descriptor = types.findDescriptorType(pt);
                    } catch (Types.FunctionDescriptorLookupError ex) {
                        checkContext.report(null, ex.getDiagnostic());
                    }
                    Env<AttrContext> localEnv = env.dup(tree);
                    JCExpression exprTree;
                    exprTree = (JCExpression)attribSpeculative(tree.getQualifierExpression(), localEnv,
                            attr.memberReferenceQualifierResult(tree), argumentAttr.withLocalCacheContext());
                    ListBuffer<Type> argtypes = new ListBuffer<>();
                    for (Type t : descriptor.getParameterTypes()) {
                        argtypes.append(Type.noType);
                    }
                    JCMemberReference mref2 = new TreeCopier<Void>(make).copy(tree);
                    mref2.expr = exprTree;
                    Symbol lookupSym =
                            rs.resolveMemberReference(localEnv, mref2, exprTree.type,
                                    tree.name, argtypes.toList(), List.nil(), descriptor, rs.arityMethodCheck,
                                    inferenceContext, rs.structuralReferenceChooser).fst;
                    switch (lookupSym.kind) {
                        case WRONG_MTH:
                        case WRONG_MTHS:
                            //note: as argtypes are erroneous types, type-errors must
                            //have been caused by arity mismatch
                            checkContext.report(tree, diags.fragment(Fragments.IncompatibleArgTypesInMref));
                            break;
                        case ABSENT_MTH:
                        case STATICERR:
                            //if no method found, or method found with wrong staticness, report better message
                            checkContext.report(tree, ((ResolveError)lookupSym).getDiagnostic(DiagnosticType.FRAGMENT,
                                    tree, exprTree.type.tsym, exprTree.type, tree.name, argtypes.toList(), List.nil()));
                            break;
                    }
                }
            }
        }

        /* This visitor looks for return statements, its analysis will determine if
         * a lambda body is void or value compatible. We must analyze return
         * statements contained in the lambda body only, thus any return statement
         * contained in an inner class or inner lambda body, should be ignored.
         */
        class LambdaBodyStructChecker extends TreeScanner {
            boolean isVoidCompatible = true;
            boolean isPotentiallyValueCompatible = true;

            @Override
            public void visitClassDef(JCClassDecl tree) {
                // do nothing
            }

            @Override
            public void visitLambda(JCLambda tree) {
                // do nothing
            }

            @Override
            public void visitNewClass(JCNewClass tree) {
                // do nothing
            }

            @Override
            public void visitReturn(JCReturn tree) {
                if (tree.expr != null) {
                    isVoidCompatible = false;
                } else {
                    isPotentiallyValueCompatible = false;
                }
            }
        }
    }

    
an empty deferred attribution context - all methods throw exceptions /** an empty deferred attribution context - all methods throw exceptions */
    final DeferredAttrContext emptyDeferredAttrContext;

    
Map a list of types possibly containing one or more deferred types
into a list of ordinary types. Each deferred type D is mapped into a type T,
where T is computed by retrieving the type that has already been
computed for D during a previous deferred attribution round of the given kind.
/**
     * Map a list of types possibly containing one or more deferred types
     * into a list of ordinary types. Each deferred type D is mapped into a type T,
     * where T is computed by retrieving the type that has already been
     * computed for D during a previous deferred attribution round of the given kind.
     */
    class DeferredTypeMap<T> extends StructuralTypeMapping<T> {
        DeferredAttrContext deferredAttrContext;

        protected DeferredTypeMap(AttrMode mode, Symbol msym, MethodResolutionPhase phase) {
            this.deferredAttrContext = new DeferredAttrContext(mode, msym, phase,
                    infer.emptyContext, emptyDeferredAttrContext, types.noWarnings);
        }

        @Override
        public Type visitType(Type t, T p) {
            if (!t.hasTag(DEFERRED)) {
                return super.visitType(t, p);
            } else {
                DeferredType dt = (DeferredType)t;
                return typeOf(dt, p);
            }
        }

        protected Type typeOf(DeferredType dt, T p) {
            switch (deferredAttrContext.mode) {
                case CHECK:
                    return dt.tree.type == null ? Type.noType : dt.tree.type;
                case SPECULATIVE:
                    return dt.speculativeType(deferredAttrContext.msym, deferredAttrContext.phase);
            }
            Assert.error();
            return null;
        }
    }

    
Specialized recovery deferred mapping.
Each deferred type D is mapped into a type T, where T is computed either by
(i) retrieving the type that has already been computed for D during a previous
attribution round (as before), or (ii) by synthesizing a new type R for D
(the latter step is useful in a recovery scenario).
/**
     * Specialized recovery deferred mapping.
     * Each deferred type D is mapped into a type T, where T is computed either by
     * (i) retrieving the type that has already been computed for D during a previous
     * attribution round (as before), or (ii) by synthesizing a new type R for D
     * (the latter step is useful in a recovery scenario).
     */
    public class RecoveryDeferredTypeMap extends DeferredTypeMap<Type> {

        public RecoveryDeferredTypeMap(AttrMode mode, Symbol msym, MethodResolutionPhase phase) {
            super(mode, msym, phase != null ? phase : MethodResolutionPhase.BOX);
        }

        @Override
        protected Type typeOf(DeferredType dt, Type pt) {
            Type owntype = super.typeOf(dt, pt);
            return owntype == Type.noType ?
                        recover(dt, pt) : owntype;
        }

        @Override
        public Type visitMethodType(Type.MethodType t, Type pt) {
            if (t.hasTag(METHOD) && deferredAttrContext.mode == AttrMode.CHECK) {
                Type mtype = deferredAttrContext.msym.type;
                mtype = mtype.hasTag(ERROR) ? ((ErrorType)mtype).getOriginalType() : null;
                if (mtype != null && mtype.hasTag(METHOD)) {
                    List<Type> argtypes1 = map(t.getParameterTypes(), mtype.getParameterTypes());
                    Type restype1 = visit(t.getReturnType(), mtype.getReturnType());
                    List<Type> thrown1 = map(t.getThrownTypes(), mtype.getThrownTypes());
                    if (argtypes1 == t.getParameterTypes() &&
                        restype1 == t.getReturnType() &&
                        thrown1 == t.getThrownTypes()) return t;
                    else return new MethodType(argtypes1, restype1, thrown1, t.tsym);
                }
            }
            return super.visitMethodType(t, pt);
        }

        
Synthesize a type for a deferred type that hasn't been previously
reduced to an ordinary type. Functional deferred types and conditionals
are mapped to themselves, in order to have a richer diagnostic
representation. Remaining deferred types are attributed using
a default expected type (j.l.Object).
/**
         * Synthesize a type for a deferred type that hasn't been previously
         * reduced to an ordinary type. Functional deferred types and conditionals
         * are mapped to themselves, in order to have a richer diagnostic
         * representation. Remaining deferred types are attributed using
         * a default expected type (j.l.Object).
         */
        private Type recover(DeferredType dt, Type pt) {
            boolean isLambdaOrMemberRef =
                    dt.tree.hasTag(REFERENCE) || dt.tree.hasTag(LAMBDA);
            boolean needsRecoveryType =
                    pt == null || (isLambdaOrMemberRef && !types.isFunctionalInterface(pt));
            Type ptRecovery = needsRecoveryType ? Type.recoveryType: pt;
            dt.check(attr.new RecoveryInfo(deferredAttrContext, ptRecovery) {
                @Override
                protected Type check(DiagnosticPosition pos, Type found) {
                    return chk.checkNonVoid(pos, super.check(pos, found));
                }
            });
            return super.visit(dt);
        }

        private List<Type> map(List<Type> ts, List<Type> pts) {
            if (ts.nonEmpty()) {
                List<Type> tail1 = map(ts.tail, pts != null ? pts.tail : null);
                Type t = visit(ts.head, pts != null && pts.nonEmpty() ? pts.head : null);
                if (tail1 != ts.tail || t != ts.head)
                    return tail1.prepend(t);
            }
            return ts;
        }
    }

    
A special tree scanner that would only visit portions of a given tree.
The set of nodes visited by the scanner can be customized at construction-time.
/**
     * A special tree scanner that would only visit portions of a given tree.
     * The set of nodes visited by the scanner can be customized at construction-time.
     */
    abstract static class FilterScanner extends com.sun.tools.javac.tree.TreeScanner {

        final Filter<JCTree> treeFilter;

        FilterScanner(final Set<JCTree.Tag> validTags) {
            this.treeFilter = t -> validTags.contains(t.getTag());
        }

        @Override
        public void scan(JCTree tree) {
            if (tree != null) {
                if (treeFilter.accepts(tree)) {
                    super.scan(tree);
                } else {
                    skip(tree);
                }
            }
        }

        
handler that is executed when a node has been discarded
/**
         * handler that is executed when a node has been discarded
         */
        void skip(JCTree tree) {}
    }

    
A tree scanner suitable for visiting the target-type dependent nodes of
a given argument expression.
/**
     * A tree scanner suitable for visiting the target-type dependent nodes of
     * a given argument expression.
     */
    static class PolyScanner extends FilterScanner {

        PolyScanner() {
            super(EnumSet.of(CONDEXPR, PARENS, LAMBDA, REFERENCE));
        }
    }

    
A tree scanner suitable for visiting the target-type dependent nodes nested
within a lambda expression body.
/**
     * A tree scanner suitable for visiting the target-type dependent nodes nested
     * within a lambda expression body.
     */
    static class LambdaReturnScanner extends FilterScanner {

        LambdaReturnScanner() {
            super(EnumSet.of(BLOCK, CASE, CATCH, DOLOOP, FOREACHLOOP,
                    FORLOOP, IF, RETURN, SYNCHRONIZED, SWITCH, TRY, WHILELOOP));
        }
    }

    
A tree scanner suitable for visiting the target-type dependent nodes nested
within a switch expression body.
/**
     * A tree scanner suitable for visiting the target-type dependent nodes nested
     * within a switch expression body.
     */
    static class SwitchExpressionScanner extends FilterScanner {

        SwitchExpressionScanner() {
            super(EnumSet.of(BLOCK, CASE, CATCH, DOLOOP, FOREACHLOOP,
                    FORLOOP, IF, SYNCHRONIZED, SWITCH, TRY, WHILELOOP, YIELD));
        }
    }

    
This visitor is used to check that structural expressions conform
to their target - this step is required as inference could end up
inferring types that make some of the nested expressions incompatible
with their corresponding instantiated target
/**
     * This visitor is used to check that structural expressions conform
     * to their target - this step is required as inference could end up
     * inferring types that make some of the nested expressions incompatible
     * with their corresponding instantiated target
     */
    class CheckStuckPolicy extends PolyScanner implements DeferredStuckPolicy, Infer.FreeTypeListener {

        Type pt;
        InferenceContext inferenceContext;
        Set<Type> stuckVars = new LinkedHashSet<>();
        Set<Type> depVars = new LinkedHashSet<>();

        @Override
        public boolean isStuck() {
            return !stuckVars.isEmpty();
        }

        @Override
        public Set<Type> stuckVars() {
            return stuckVars;
        }

        @Override
        public Set<Type> depVars() {
            return depVars;
        }

        public CheckStuckPolicy(ResultInfo resultInfo, DeferredType dt) {
            this.pt = resultInfo.pt;
            this.inferenceContext = resultInfo.checkContext.inferenceContext();
            scan(dt.tree);
            if (!stuckVars.isEmpty()) {
                resultInfo.checkContext.inferenceContext()
                        .addFreeTypeListener(List.from(stuckVars), this);
            }
        }

        @Override
        public void typesInferred(InferenceContext inferenceContext) {
            stuckVars.clear();
        }

        @Override
        public void visitLambda(JCLambda tree) {
            if (inferenceContext.inferenceVars().contains(pt)) {
                stuckVars.add(pt);
            }
            if (!types.isFunctionalInterface(pt)) {
                return;
            }
            Type descType = types.findDescriptorType(pt);
            List<Type> freeArgVars = inferenceContext.freeVarsIn(descType.getParameterTypes());
            if (tree.paramKind == JCLambda.ParameterKind.IMPLICIT &&
                    freeArgVars.nonEmpty()) {
                stuckVars.addAll(freeArgVars);
                depVars.addAll(inferenceContext.freeVarsIn(descType.getReturnType()));
            }
            scanLambdaBody(tree, descType.getReturnType());
        }

        @Override
        public void visitReference(JCMemberReference tree) {
            scan(tree.expr);
            if (inferenceContext.inferenceVars().contains(pt)) {
                stuckVars.add(pt);
                return;
            }
            if (!types.isFunctionalInterface(pt)) {
                return;
            }

            Type descType = types.findDescriptorType(pt);
            List<Type> freeArgVars = inferenceContext.freeVarsIn(descType.getParameterTypes());
            if (freeArgVars.nonEmpty() &&
                    tree.getOverloadKind() != JCMemberReference.OverloadKind.UNOVERLOADED) {
                stuckVars.addAll(freeArgVars);
                depVars.addAll(inferenceContext.freeVarsIn(descType.getReturnType()));
            }
        }

        void scanLambdaBody(JCLambda lambda, final Type pt) {
            if (lambda.getBodyKind() == JCTree.JCLambda.BodyKind.EXPRESSION) {
                Type prevPt = this.pt;
                try {
                    this.pt = pt;
                    scan(lambda.body);
                } finally {
                    this.pt = prevPt;
                }
            } else {
                LambdaReturnScanner lambdaScanner = new LambdaReturnScanner() {
                    @Override
                    public void visitReturn(JCReturn tree) {
                        if (tree.expr != null) {
                            Type prevPt = CheckStuckPolicy.this.pt;
                            try {
                                CheckStuckPolicy.this.pt = pt;
                                CheckStuckPolicy.this.scan(tree.expr);
                            } finally {
                                CheckStuckPolicy.this.pt = prevPt;
                            }
                        }
                    }
                };
                lambdaScanner.scan(lambda.body);
            }
        }
    }

    
This visitor is used to check that structural expressions conform
to their target - this step is required as inference could end up
inferring types that make some of the nested expressions incompatible
with their corresponding instantiated target
/**
     * This visitor is used to check that structural expressions conform
     * to their target - this step is required as inference could end up
     * inferring types that make some of the nested expressions incompatible
     * with their corresponding instantiated target
     */
    class OverloadStuckPolicy extends CheckStuckPolicy implements DeferredStuckPolicy {

        boolean stuck;

        @Override
        public boolean isStuck() {
            return super.isStuck() || stuck;
        }

        public OverloadStuckPolicy(ResultInfo resultInfo, DeferredType dt) {
            super(resultInfo, dt);
        }

        @Override
        public void visitLambda(JCLambda tree) {
            super.visitLambda(tree);
            if (tree.paramKind == JCLambda.ParameterKind.IMPLICIT) {
                stuck = true;
            }
        }

        @Override
        public void visitReference(JCMemberReference tree) {
            super.visitReference(tree);
            if (tree.getOverloadKind() != JCMemberReference.OverloadKind.UNOVERLOADED) {
                stuck = true;
            }
        }
    }

    
Mode of attribution (used in AttrContext).
/**
     * Mode of attribution (used in AttrContext).
     */
    enum AttributionMode {
        
Normal, non-speculative, attribution./**Normal, non-speculative, attribution.*/
        FULL(false, true),
        
Speculative attribution on behalf of an Analyzer./**Speculative attribution on behalf of an Analyzer.*/
        ATTRIB_TO_TREE(true, true),
        
Speculative attribution on behalf of an Analyzer./**Speculative attribution on behalf of an Analyzer.*/
        ANALYZER(true, false),
        
Speculative attribution./**Speculative attribution.*/
        SPECULATIVE(true, false);

        AttributionMode(boolean isSpeculative, boolean recover) {
            this.isSpeculative = isSpeculative;
            this.recover = recover;
        }

        boolean isSpeculative() {
            return isSpeculative;
        }

        boolean recover() {
            return recover;
        }

        final boolean isSpeculative;
        final boolean recover;
    }
}