javac/comp/DeferredAttr.java

/*
 * Copyright (c) 2012, 2017, 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 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.StructuralTypeMapping;
import com.sun.tools.javac.code.Types.TypeMapping;
import com.sun.tools.javac.comp.ArgumentAttr.LocalCacheContext;
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.code.Symbol.*;
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 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.Function;

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

import static com.sun.tools.javac.code.TypeTag.*;
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.
 *
 *  <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 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;

    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);
        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);
        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).NewClass(encl, typeargs, clazz, args, def);
                    } 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) {
                            super.setOverloadKind(overloadKind);
                            if (t.getOverloadKind() == null) {
                                t.setOverloadKind(overloadKind);
                            }
                        }
                    };
                    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 */
    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.
     */
    public class DeferredType extends Type {

        public JCExpression tree;
        Env<AttrContext> env;
        AttrMode mode;
        boolean pertinentToApplicability = true;
        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.
         */
        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
             */
            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
             */
            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
         */
        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.
         */
        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()) {
                pertinentToApplicability = false;
                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.
     */
    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).
         */
        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.
     */
    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.
     */
    interface DeferredStuckPolicy {
        /**
         * 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.
         */
        Set<Type> stuckVars();
        /**
         * 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.
     */
    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
     */
    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.
         */
        SPECULATIVE,
        /**
         * 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 {@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.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.
     */
    JCTree attribSpeculative(JCTree tree, Env<AttrContext> env, ResultInfo resultInfo) {
        return attribSpeculative(tree, env, resultInfo, treeCopier,
                (newTree)->new DeferredAttrDiagHandler(log, newTree), null);
    }

    JCTree attribSpeculative(JCTree tree, Env<AttrContext> env, ResultInfo resultInfo, LocalCacheContext localCache) {
        return attribSpeculative(tree, env, resultInfo, treeCopier,
                (newTree)->new DeferredAttrDiagHandler(log, newTree), localCache);
    }

    <Z> JCTree attribSpeculative(JCTree tree, Env<AttrContext> env, ResultInfo resultInfo, TreeCopier<Z> deferredCopier,
                                 Function<JCTree, DeferredDiagnosticHandler> diagHandlerCreator,
                                 LocalCacheContext localCache) {
        final JCTree newTree = deferredCopier.copy(tree);
        Env<AttrContext> speculativeEnv = env.dup(newTree, env.info.dup(env.info.scope.dupUnshared(env.info.scope.owner)));
        speculativeEnv.info.isSpeculative = true;
        Log.DeferredDiagnosticHandler deferredDiagnosticHandler = diagHandlerCreator.apply(newTree);
        try {
            attr.attribTree(newTree, speculativeEnv, resultInfo);
            return newTree;
        } finally {
            new UnenterScanner(env.toplevel.modle).scan(newTree);
            log.popDiagnosticHandler(deferredDiagnosticHandler);
            if (localCache != null) {
                localCache.leave();
            }
        }
    }
    //where

        class UnenterScanner extends TreeScanner {
            private final ModuleSymbol msym;

            public UnenterScanner(ModuleSymbol msym) {
                this.msym = msym;
            }

            @Override
            public void visitClassDef(JCClassDecl tree) {
                ClassSymbol csym = tree.sym;
                //if something went wrong during method applicability check
                //it is possible that nested expressions inside argument expression
                //are left unchecked - in such cases there's nothing to clean up.
                if (csym == null) return;
                typeEnvs.remove(csym);
                chk.removeCompiled(csym);
                chk.clearLocalClassNameIndexes(csym);
                syms.removeClass(msym, csym.flatname);
                super.visitClassDef(tree);
            }
        }

        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.
     */
    class DeferredAttrContext {

        /** attribution mode */
        final AttrMode mode;

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

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

        /** inference context */
        final InferenceContext inferenceContext;

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

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

        /** 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.
         */
        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.
         */
        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 contraints are too tight for progess 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. The chosen node is the first strongly connected
         * component containing exactly one node found in the dependency graph induced by deferred nodes.
         * If no such component is found, the first deferred node is returned.
         */
        DeferredAttrNode pickDeferredNode() {
            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
            //the intersection between A's input variable and B's output variable is non-empty.
            for (StuckNode sn1 : nodes) {
                for (Type t : sn1.data.deferredStuckPolicy.stuckVars()) {
                    for (StuckNode sn2 : nodes) {
                        if (sn1 != sn2 && sn2.data.deferredStuckPolicy.depVars().contains(t)) {
                            sn1.deps.add(sn2);
                        }
                    }
                }
            }
            //compute tarjan on the stuck graph
            List<? extends StuckNode> csn = GraphUtils.tarjan(nodes).get(0);
            return csn.length() == 1 ? csn.get(0).data : deferredAttrNodes.get(0);
        }

        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 DeferredAttrNode {

        /** underlying deferred type */
        DeferredType dt;

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

        /** 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.
         */
        @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
         */
        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 visitReference(JCMemberReference tree) {
                Assert.checkNonNull(tree.getOverloadKind());
                Check.CheckContext checkContext = resultInfo.checkContext;
                Type pt = resultInfo.pt;
                if (!inferenceContext.inferencevars.contains(pt)) {
                    try {
                        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 : types.findDescriptorType(pt).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(), 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 */
    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.
     */
    class DeferredTypeMap extends StructuralTypeMapping<Void> {
        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, Void _unused) {
            if (!t.hasTag(DEFERRED)) {
                return super.visitType(t, null);
            } else {
                DeferredType dt = (DeferredType)t;
                return typeOf(dt);
            }
        }

        protected Type typeOf(DeferredType dt) {
            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).
     */
    public class RecoveryDeferredTypeMap extends DeferredTypeMap {

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

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

        /**
         * 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) {
            dt.check(attr.new RecoveryInfo(deferredAttrContext) {
                @Override
                protected Type check(DiagnosticPosition pos, Type found) {
                    return chk.checkNonVoid(pos, super.check(pos, found));
                }
            });
            return super.visit(dt);
        }
    }

    /**
     * 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
         */
        void skip(JCTree tree) {}
    }

    /**
     * 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.
     */
    static class LambdaReturnScanner extends FilterScanner {

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

    /**
     * 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.OVERLOADED) {
                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
     */
    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.OVERLOADED) {
                stuck = true;
            }
        }
    }
}