Mercurial > jdk8-mips64-public > langtools / file revision

 /*

  * Copyright (c) 2012, 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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  * questions.

  */

 package com.sun.tools.javac.comp;

 import com.sun.tools.javac.code.*;

 import com.sun.tools.javac.tree.*;

 import com.sun.tools.javac.util.*;

 import com.sun.tools.javac.code.Symbol.*;

 import com.sun.tools.javac.code.Type.*;

 import com.sun.tools.javac.comp.Attr.ResultInfo;

 import com.sun.tools.javac.comp.Infer.InferenceContext;

 import com.sun.tools.javac.comp.Resolve.MethodResolutionPhase;

 import com.sun.tools.javac.tree.JCTree.*;

 import javax.tools.JavaFileObject;

 import java.util.ArrayList;

 import java.util.EnumSet;

 import java.util.LinkedHashSet;

 import java.util.Map;

 import java.util.Queue;

 import java.util.Set;

 import java.util.WeakHashMap;

 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<DeferredAttr>();

     final Attr attr;

     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 Types types;

     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);

         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);

         Names names = Names.instance(context);

         stuckTree = make.Ident(names.empty).setType(Type.noType);

     }

     /** 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;

         SpeculativeCache speculativeCache;

         DeferredType(JCExpression tree, Env<AttrContext> env) {

             super(DEFERRED, null);

             this.tree = tree;

             this.env = env.dup(tree, env.info.dup());

             this.speculativeCache = new SpeculativeCache();

         }

         /**

          * 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<Symbol, List<Entry>>();

             class Entry {

                 JCTree speculativeTree;

                 Resolve.MethodResolutionPhase phase;

                 public Entry(JCTree speculativeTree, MethodResolutionPhase phase) {

                     this.speculativeTree = speculativeTree;

                     this.phase = phase;

                 }

                 boolean matches(Resolve.MethodResolutionPhase phase) {

                     return this.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(Symbol msym, JCTree speculativeTree, MethodResolutionPhase phase) {

                 List<Entry> entries = cache.get(msym);

                 if (entries == null) {

                     entries = List.nil();

                 }

                 cache.put(msym, entries.prepend(new Entry(speculativeTree, phase)));

             }

         }

         /**

          * 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;

         }

         /**

          * 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) {

             return check(resultInfo, stuckVars(tree, env, resultInfo), basicCompleter);

         }

         Type check(ResultInfo resultInfo, List<Type> stuckVars, DeferredTypeCompleter deferredTypeCompleter) {

             DeferredAttrContext deferredAttrContext =

                     resultInfo.checkContext.deferredAttrContext();

             Assert.check(deferredAttrContext != emptyDeferredAttrContext);

             if (stuckVars.nonEmpty()) {

                 deferredAttrContext.addDeferredAttrNode(this, resultInfo, stuckVars);

                 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:

                     Assert.check(dt.mode == null ||

                             (dt.mode == AttrMode.SPECULATIVE &&

                             dt.speculativeType(deferredAttrContext.msym, deferredAttrContext.phase).hasTag(NONE)));

                     JCTree speculativeTree = attribSpeculative(dt.tree, dt.env, resultInfo);

                     dt.speculativeCache.put(deferredAttrContext.msym, speculativeTree, deferredAttrContext.phase);

                     return speculativeTree.type;

                 case CHECK:

                     Assert.check(dt.mode != null);

                     return attr.attribTree(dt.tree, dt.env, resultInfo);

             }

             Assert.error();

             return null;

         }

     };

     DeferredTypeCompleter dummyCompleter = new DeferredTypeCompleter() {

         public Type complete(DeferredType dt, ResultInfo resultInfo, DeferredAttrContext deferredAttrContext) {

             Assert.check(deferredAttrContext.mode == AttrMode.CHECK);

             return dt.tree.type = Type.noType;

         }

     };

     /**

      * 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;

     }

     /**

      * 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) {

         final JCTree newTree = new TreeCopier<Object>(make).copy(tree);

         Env<AttrContext> speculativeEnv = env.dup(newTree, env.info.dup(env.info.scope.dupUnshared()));

         speculativeEnv.info.scope.owner = env.info.scope.owner;

         Log.DeferredDiagnosticHandler deferredDiagnosticHandler =

                 new Log.DeferredDiagnosticHandler(log, new Filter<JCDiagnostic>() {

             public boolean accepts(final JCDiagnostic d) {

                 class PosScanner extends TreeScanner {

                     boolean found = false;

                     @Override

                     public void scan(JCTree tree) {

                         if (tree != null &&

                                 tree.pos() == d.getDiagnosticPosition()) {

                             found = true;

                         }

                         super.scan(tree);

                     }

                 };

                 PosScanner posScanner = new PosScanner();

                 posScanner.scan(newTree);

                 return posScanner.found;

             }

         });

         try {

             attr.attribTree(newTree, speculativeEnv, resultInfo);

             unenterScanner.scan(newTree);

             return newTree;

         } catch (Abort ex) {

             //if some very bad condition occurred during deferred attribution

             //we should dump all errors before killing javac

             deferredDiagnosticHandler.reportDeferredDiagnostics();

             throw ex;

         } finally {

             unenterScanner.scan(newTree);

             log.popDiagnosticHandler(deferredDiagnosticHandler);

         }

     }

     //where

         protected TreeScanner unenterScanner = new TreeScanner() {

             @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;

                 enter.typeEnvs.remove(csym);

                 chk.compiled.remove(csym.flatname);

                 syms.classes.remove(csym.flatname);

                 super.visitClassDef(tree);

             }

         };

     /**

      * 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<DeferredAttrNode>();

         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, List<Type> stuckVars) {

             deferredAttrNodes.add(new DeferredAttrNode(dt, resultInfo, stuckVars));

         }

         /**

          * 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()) {

                 Set<Type> stuckVars = new LinkedHashSet<Type>();

                 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)) {

                         stuckVars.addAll(deferredAttrNode.stuckVars);

                     } else {

                         deferredAttrNodes.remove(deferredAttrNode);

                         progress = true;

                     }

                 }

                 if (!progress) {

                     //remove all variables that have already been instantiated

                     //from the list of stuck variables

                     inferenceContext.solveAny(List.from(stuckVars), warn);

                     inferenceContext.notifyChange();

                 }

             }

         }

     }

     /**

      * Class representing a deferred attribution node. It keeps track of

      * a deferred type, along with the expected target type information.

      */

     class DeferredAttrNode implements Infer.FreeTypeListener {

         /** underlying deferred type */

         DeferredType dt;

         /** underlying target type information */

         ResultInfo resultInfo;

         /** list of uninferred inference variables causing this node to be stuck */

         List<Type> stuckVars;

         DeferredAttrNode(DeferredType dt, ResultInfo resultInfo, List<Type> stuckVars) {

             this.dt = dt;

             this.resultInfo = resultInfo;

             this.stuckVars = stuckVars;

             if (!stuckVars.isEmpty()) {

                 resultInfo.checkContext.inferenceContext().addFreeTypeListener(stuckVars, this);

             }

         }

         @Override

         public void typesInferred(InferenceContext inferenceContext) {

             stuckVars = List.nil();

             resultInfo = resultInfo.dup(inferenceContext.asInstType(resultInfo.pt));

         }

         /**

          * Process a deferred attribution node.

          * Invariant: a stuck node cannot be processed.

          */

         @SuppressWarnings("fallthrough")

         boolean process(DeferredAttrContext deferredAttrContext) {

             switch (deferredAttrContext.mode) {

                 case SPECULATIVE:

                     dt.check(resultInfo, List.<Type>nil(), new StructuralStuckChecker());

                     return true;

                 case CHECK:

                     if (stuckVars.nonEmpty()) {

                         //stuck expression - see if we can propagate

                         if (deferredAttrContext.parent != emptyDeferredAttrContext &&

                                 Type.containsAny(deferredAttrContext.parent.inferenceContext.inferencevars, List.from(stuckVars))) {

                             deferredAttrContext.parent.deferredAttrNodes.add(this);

                             dt.check(resultInfo, List.<Type>nil(), dummyCompleter);

                             return true;

                         } else {

                             return false;

                         }

                     } else {

                         dt.check(resultInfo, stuckVars, 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.dup(dt.tree, dt.env.info.dup());

                 dt.tree.accept(this);

                 dt.speculativeCache.put(deferredAttrContext.msym, stuckTree, deferredAttrContext.phase);

                 return Type.noType;

             }

             @Override

             public void visitLambda(JCLambda tree) {

                 Check.CheckContext checkContext = resultInfo.checkContext;

                 Type pt = resultInfo.pt;

                 if (inferenceContext.inferencevars.contains(pt)) {

                     //ok

                     return;

                 } else {

                     //must be a functional descriptor

                     try {

                         Type desc = types.findDescriptorType(pt);

                         if (desc.getParameterTypes().length() != tree.params.length()) {

                             checkContext.report(tree, diags.fragment("incompatible.arg.types.in.lambda"));

                         }

                     } catch (Types.FunctionDescriptorLookupError ex) {

                         checkContext.report(null, ex.getDiagnostic());

                     }

                 }

             }

             @Override

             public void visitNewClass(JCNewClass tree) {

                 //do nothing

             }

             @Override

             public void visitApply(JCMethodInvocation tree) {

                 //do nothing

             }

             @Override

             public void visitReference(JCMemberReference tree) {

                 Check.CheckContext checkContext = resultInfo.checkContext;

                 Type pt = resultInfo.pt;

                 if (inferenceContext.inferencevars.contains(pt)) {

                     //ok

                     return;

                 } else {

                     try {

                         types.findDescriptorType(pt);

                     } catch (Types.FunctionDescriptorLookupError ex) {

                         checkContext.report(null, ex.getDiagnostic());

                     }

                     JCExpression exprTree = (JCExpression)attribSpeculative(tree.getQualifierExpression(), env,

                             attr.memberReferenceQualifierResult(tree));

                     ListBuffer<Type> argtypes = ListBuffer.lb();

                     for (Type t : types.findDescriptorType(pt).getParameterTypes()) {

                         argtypes.append(syms.errType);

                     }

                     JCMemberReference mref2 = new TreeCopier<Void>(make).copy(tree);

                     mref2.expr = exprTree;

                     Pair<Symbol, ?> lookupRes =

                             rs.resolveMemberReference(tree, env, mref2, exprTree.type, tree.name, argtypes.toList(), null, true);

                     switch (lookupRes.fst.kind) {

                         //note: as argtypes are erroneous types, type-errors must

                         //have been caused by arity mismatch

                         case Kinds.ABSENT_MTH:

                         case Kinds.WRONG_MTH:

                         case Kinds.WRONG_MTHS:

                         case Kinds.STATICERR:

                         case Kinds.MISSING_ENCL:

                            checkContext.report(null, diags.fragment("incompatible.arg.types.in.mref"));

                     }

                 }

             }

         }

     }

     /** an empty deferred attribution context - all methods throw exceptions */

     final DeferredAttrContext emptyDeferredAttrContext =

             new DeferredAttrContext(AttrMode.CHECK, null, MethodResolutionPhase.BOX, null, null, null) {

                 @Override

                 void addDeferredAttrNode(DeferredType dt, ResultInfo ri, List<Type> stuckVars) {

                     Assert.error("Empty deferred context!");

                 }

                 @Override

                 void complete() {

                     Assert.error("Empty deferred context!");

                 }

             };

     /**

      * 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 Type.Mapping {

         DeferredAttrContext deferredAttrContext;

         protected DeferredTypeMap(AttrMode mode, Symbol msym, MethodResolutionPhase phase) {

             super(String.format("deferredTypeMap[%s]", mode));

             this.deferredAttrContext = new DeferredAttrContext(mode, msym, phase,

                     infer.emptyContext, emptyDeferredAttrContext, types.noWarnings);

         }

         protected boolean validState(DeferredType dt) {

             return dt.mode != null &&

                     deferredAttrContext.mode.ordinal() <= dt.mode.ordinal();

         }

         @Override

         public Type apply(Type t) {

             if (!t.hasTag(DEFERRED)) {

                 return t.map(this);

             } else {

                 DeferredType dt = (DeferredType)t;

                 Assert.check(validState(dt));

                 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;

         }

         @Override

         protected boolean validState(DeferredType dt) {

             return true;

         }

         /**

          * 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));

             return super.apply(dt);

         }

     }

     /**

      * Retrieves the list of inference variables that need to be inferred before

      * an AST node can be type-checked

      */

     @SuppressWarnings("fallthrough")

     List<Type> stuckVars(JCTree tree, Env<AttrContext> env, ResultInfo resultInfo) {

                 if (resultInfo.pt.hasTag(NONE) || resultInfo.pt.isErroneous()) {

             return List.nil();

         } else {

             return stuckVarsInternal(tree, resultInfo.pt, resultInfo.checkContext.inferenceContext());

         }

     }

     //where

         private List<Type> stuckVarsInternal(JCTree tree, Type pt, Infer.InferenceContext inferenceContext) {

             StuckChecker sc = new StuckChecker(pt, inferenceContext);

             sc.scan(tree);

             return List.from(sc.stuckVars);

         }

     /**

      * 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 TreeScanner {

         final Filter<JCTree> treeFilter;

         FilterScanner(final Set<JCTree.Tag> validTags) {

             this.treeFilter = new Filter<JCTree>() {

                 public boolean accepts(JCTree t) {

                     return 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

          */

         abstract 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));

         }

         @Override

         void skip(JCTree tree) {

             //do nothing

         }

     }

     /**

      * 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, RETURN, SYNCHRONIZED, SWITCH, TRY, WHILELOOP));

         }

         @Override

         void skip(JCTree tree) {

             //do nothing

         }

     }

     /**

      * 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 StuckChecker extends PolyScanner {

         Type pt;

         Infer.InferenceContext inferenceContext;

         Set<Type> stuckVars = new LinkedHashSet<Type>();

         StuckChecker(Type pt, Infer.InferenceContext inferenceContext) {

             this.pt = pt;

             this.inferenceContext = inferenceContext;

         }

         @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);

             }

             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());

             stuckVars.addAll(freeArgVars);

         }

         void scanLambdaBody(JCLambda lambda, final Type pt) {

             if (lambda.getBodyKind() == JCTree.JCLambda.BodyKind.EXPRESSION) {

                 stuckVars.addAll(stuckVarsInternal(lambda.body, pt, inferenceContext));

             } else {

                 LambdaReturnScanner lambdaScanner = new LambdaReturnScanner() {

                     @Override

                     public void visitReturn(JCReturn tree) {

                         if (tree.expr != null) {

                             stuckVars.addAll(stuckVarsInternal(tree.expr, pt, inferenceContext));

                         }

                     }

                 };

                 lambdaScanner.scan(lambda.body);

             }

         }

     }

 }