[es8] More spec compliant syntactic tail calls implementation.

src/parsing/parser-base.h

 // Copyright 2012 the V8 project authors. All rights reserved.
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
 #ifndef V8_PARSING_PARSER_BASE_H
 #define V8_PARSING_PARSER_BASE_H
 
 #include "src/ast/scopes.h"
 #include "src/bailout-reason.h"
 #include "src/hashmap.h"
@@ skipping 172 matching lines @@
 
   struct DestructuringAssignment {
    public:
     DestructuringAssignment(ExpressionT expression, Scope* scope)
         : assignment(expression), scope(scope) {}
 
     ExpressionT assignment;
     Scope* scope;
   };
 
-  struct TailCallExpression {
-    TailCallExpression(ExpressionT expression, int pos)
-        : expression(expression), pos(pos) {}
+  class TailCallExpressionList {
+   public:
+    explicit TailCallExpressionList(Zone* zone)
+        : zone_(zone), expressions_(0, zone) {}
 
-    ExpressionT expression;
-    int pos;
+    const ZoneList<ExpressionT>& expressions() const { return expressions_; }
+    const Scanner::Location& location() const { return loc_; }
+
+    bool is_empty() const { return expressions_.is_empty(); }
+
+    void Swap(TailCallExpressionList& other) {
+      expressions_.Swap(&other.expressions_);
+      std::swap(loc_, other.loc_);
+    }
+
+    void Add(ExpressionT expr, const Scanner::Location& loc) {
+      if (expressions_.is_empty()) loc_ = loc;
+      expressions_.Add(expr, zone_);
+    }
+
+    void Append(const TailCallExpressionList& other) {
+      if (expressions_.is_empty()) loc_ = other.loc_;
+      expressions_.AddAll(other.expressions_, zone_);
+    }
+
+   private:
+    Zone* zone_;
+    ZoneList<ExpressionT> expressions_;
+    Scanner::Location loc_;
   };
 
   // Defines whether tail call expressions are allowed or not.
   enum class ReturnExprContext {
-    // Tail call expressions are allowed.
-    kNormal,
-    // Tail call expressions are not allowed.
+    // We are inside return statement which is allowed to contain tail call
+    // expressions. Tail call expressions are allowed.
+    kInsideValidReturnStatement,
+
+    // We are inside a block in which tail call expressions are allowed but
+    // not yet inside a return statement.
+    kInsideValidBlock,
+
+    // Tail call expressions are not allowed in the following blocks.
     kInsideTryBlock,
     kInsideForInOfBody,
   };
 
   class FunctionState BASE_EMBEDDED {
    public:
     FunctionState(FunctionState** function_state_stack, Scope** scope_stack,
                   Scope* scope, FunctionKind kind,
                   typename Traits::Type::Factory* factory);
     ~FunctionState();
@@ skipping 41 matching lines @@
       return generator_object_variable_;
     }
 
     typename Traits::Type::Factory* factory() { return factory_; }
 
     const List<DestructuringAssignment>& destructuring_assignments_to_rewrite()
         const {
       return destructuring_assignments_to_rewrite_;
     }
 
-    List<TailCallExpression>& expressions_in_tail_position() {
-      return expressions_in_tail_position_;
+    TailCallExpressionList& tail_call_expressions() {
+      return tail_call_expressions_;
     }
-    void AddExpressionInTailPosition(ExpressionT expression, int pos) {
-      if (return_expr_context() == ReturnExprContext::kNormal) {
-        expressions_in_tail_position_.Add(TailCallExpression(expression, pos));
+    void AddExpressionInTailPosition(ExpressionT expression,
+                                     const Scanner::Location& loc) {
+      // If only FLAG_harmony_explicit_tailcalls is enabled then expression
+      // must be a Call expression.
+      DCHECK(FLAG_harmony_tailcalls || !FLAG_harmony_explicit_tailcalls ||
+             expression->IsCall());
+      if (return_expr_context() ==
+          ReturnExprContext::kInsideValidReturnStatement) {
+        tail_call_expressions_.Add(expression, loc);
       }
     }
 
     ReturnExprContext return_expr_context() const {
       return return_expr_context_;
     }
     void set_return_expr_context(ReturnExprContext context) {
       return_expr_context_ = context;
     }
 
@@ skipping 42 matching lines @@
     // is used by yield expressions and return statements. It is not necessary
     // for generator functions to have this variable set.
     Variable* generator_object_variable_;
 
     FunctionState** function_state_stack_;
     FunctionState* outer_function_state_;
     Scope** scope_stack_;
     Scope* outer_scope_;
 
     List<DestructuringAssignment> destructuring_assignments_to_rewrite_;
-    List<TailCallExpression> expressions_in_tail_position_;
+    TailCallExpressionList tail_call_expressions_;
     ReturnExprContext return_expr_context_;
     ZoneList<ExpressionT> non_patterns_to_rewrite_;
 
     typename Traits::Type::Factory* factory_;
 
     // If true, the next (and immediately following) function literal is
     // preceded by a parenthesis.
     bool next_function_is_parenthesized_;
 
     // The value of the parents' next_function_is_parenthesized_, as it applies
     // to this function. Filled in by constructor.
     bool this_function_is_parenthesized_;
 
     friend class ParserTraits;
     friend class PreParserTraits;
     friend class Checkpoint;
   };
 
   // This scope sets current ReturnExprContext to given value.
   class ReturnExprScope {
    public:
     explicit ReturnExprScope(FunctionState* function_state,
                              ReturnExprContext return_expr_context)
         : function_state_(function_state),
           sav_return_expr_context_(function_state->return_expr_context()) {
-      function_state->set_return_expr_context(return_expr_context);
+      // Don't update context if we are requested to enable tail call
+      // expressions but current block does not allow them.
+      if (return_expr_context !=
+              ReturnExprContext::kInsideValidReturnStatement ||
+          sav_return_expr_context_ == ReturnExprContext::kInsideValidBlock) {
+        function_state->set_return_expr_context(return_expr_context);
+      }
     }
     ~ReturnExprScope() {
       function_state_->set_return_expr_context(sav_return_expr_context_);
     }
 
    private:
     FunctionState* function_state_;
     ReturnExprContext sav_return_expr_context_;
   };
 
   // Collects all return expressions at tail call position in this scope
   // to a separate list.
   class CollectExpressionsInTailPositionToListScope {
    public:
     CollectExpressionsInTailPositionToListScope(FunctionState* function_state,
-                                                List<TailCallExpression>* list)
+                                                TailCallExpressionList* list)
         : function_state_(function_state), list_(list) {
-      function_state->expressions_in_tail_position().Swap(list_);
+      function_state->tail_call_expressions().Swap(*list_);
     }
     ~CollectExpressionsInTailPositionToListScope() {
-      function_state_->expressions_in_tail_position().Swap(list_);
+      function_state_->tail_call_expressions().Swap(*list_);
     }
 
    private:
     FunctionState* function_state_;
-    List<TailCallExpression>* list_;
+    TailCallExpressionList* list_;
   };
 
   // Annoyingly, arrow functions first parse as comma expressions, then when we
   // see the => we have to go back and reinterpret the arguments as being formal
   // parameters.  To do so we need to reset some of the parser state back to
   // what it was before the arguments were first seen.
   class Checkpoint BASE_EMBEDDED {
    public:
     explicit Checkpoint(ParserBase* parser) {
       function_state_ = parser->function_state_;
@@ skipping 237 matching lines @@
     Traits::ReportMessageAt(source_location, message, arg, error_type);
   }
 
   void ReportMessageAt(Scanner::Location location,
                        MessageTemplate::Template message,
                        ParseErrorType error_type = kSyntaxError) {
     Traits::ReportMessageAt(location, message, reinterpret_cast<const char*>(0),
                             error_type);
   }
 
-  void ReportIllegalTailCallAt(int pos, ReturnExprContext return_expr_context) {
-    Scanner::Location loc(pos, pos + 1);
-    MessageTemplate::Template msg = MessageTemplate::kNone;
-    switch (return_expr_context) {
-      case ReturnExprContext::kNormal:
-        UNREACHABLE();
-        return;
-      case ReturnExprContext::kInsideTryBlock:
-        msg = MessageTemplate::kTailCallInTryBlock;
-        break;
-      case ReturnExprContext::kInsideForInOfBody:
-        msg = MessageTemplate::kTailCallInForInOf;
-        break;
-    }
-    ReportMessageAt(loc, msg);
-  }
-
   void GetUnexpectedTokenMessage(
       Token::Value token, MessageTemplate::Template* message,
       Scanner::Location* location, const char** arg,
       MessageTemplate::Template default_ = MessageTemplate::kUnexpectedToken);
 
   void ReportUnexpectedToken(Token::Value token);
   void ReportUnexpectedTokenAt(
       Scanner::Location location, Token::Value token,
       MessageTemplate::Template message = MessageTemplate::kUnexpectedToken);
 
@@ skipping 80 matching lines @@
     }
   }
 
   void ValidateLetPattern(const ExpressionClassifier* classifier, bool* ok) {
     if (!classifier->is_valid_let_pattern()) {
       ReportClassifierError(classifier->let_pattern_error());
       *ok = false;
     }
   }
 
+  void ValidateTailCallExpression(const ExpressionClassifier* classifier,
+                                  bool* ok) {
+    if (FLAG_harmony_explicit_tailcalls &&
+        classifier->has_tail_call_expression()) {

[rossberg, 2016/05/02 10:59:40]

I'm confused, why is it an error if it _has_ a tail call? Shouldn't it be the
other way round?

[Igor Sheludko, 2016/05/04 10:28:55]

Updated confusing function name.

+      ReportClassifierError(classifier->tail_call_expression_error());
+      *ok = false;
+    }
+  }
+
   void ExpressionUnexpectedToken(ExpressionClassifier* classifier) {
     MessageTemplate::Template message = MessageTemplate::kUnexpectedToken;
     const char* arg;
     Scanner::Location location = scanner()->peek_location();
     GetUnexpectedTokenMessage(peek(), &message, &location, &arg);
     classifier->RecordExpressionError(location, message, arg);
   }
 
   void BindingPatternUnexpectedToken(ExpressionClassifier* classifier) {
     MessageTemplate::Template message = MessageTemplate::kUnexpectedToken;
@@ skipping 13 matching lines @@
 
   void FormalParameterInitializerUnexpectedToken(
       ExpressionClassifier* classifier) {
     MessageTemplate::Template message = MessageTemplate::kUnexpectedToken;
     const char* arg;
     Scanner::Location location = scanner()->peek_location();
     GetUnexpectedTokenMessage(peek(), &message, &location, &arg);
     classifier->RecordFormalParameterInitializerError(location, message, arg);
   }
 
+  enum AcceptTailCallExpression { kDontAcceptTCE, kAcceptTCE };
+
   // Recursive descent functions:
 
   // Parses an identifier that is valid for the current scope, in particular it
   // fails on strict mode future reserved keywords in a strict scope. If
   // allow_eval_or_arguments is kAllowEvalOrArguments, we allow "eval" or
   // "arguments" as identifier even in strict mode (this is needed in cases like
   // "var foo = eval;").
   IdentifierT ParseIdentifier(AllowRestrictedIdentifiers, bool* ok);
   IdentifierT ParseAndClassifyIdentifier(ExpressionClassifier* classifier,
                                          bool* ok);
@@ skipping 33 matching lines @@
       ExpressionClassifier* classifier, IdentifierT* name, bool* ok);
   typename Traits::Type::ExpressionList ParseArguments(
       Scanner::Location* first_spread_pos, ExpressionClassifier* classifier,
       bool* ok);
 
   ExpressionT ParseAssignmentExpression(bool accept_IN,
                                         ExpressionClassifier* classifier,
                                         bool* ok);
   ExpressionT ParseYieldExpression(bool accept_IN,
                                    ExpressionClassifier* classifier, bool* ok);
+  ExpressionT ParseTailCallExpression(ExpressionClassifier* classifier,
+                                      bool* ok);
   ExpressionT ParseConditionalExpression(bool accept_IN,
                                          ExpressionClassifier* classifier,
                                          bool* ok);
   ExpressionT ParseBinaryExpression(int prec, bool accept_IN,
                                     ExpressionClassifier* classifier, bool* ok);
   ExpressionT ParseUnaryExpression(ExpressionClassifier* classifier, bool* ok);
   ExpressionT ParsePostfixExpression(ExpressionClassifier* classifier,
                                      bool* ok);
-  ExpressionT ParseLeftHandSideExpression(ExpressionClassifier* classifier,
+  ExpressionT ParseLeftHandSideExpression(AcceptTailCallExpression accept_TCE,
+                                          ExpressionClassifier* classifier,
                                           bool* ok);
   ExpressionT ParseMemberWithNewPrefixesExpression(
       ExpressionClassifier* classifier, bool* ok);
   ExpressionT ParseMemberExpression(ExpressionClassifier* classifier, bool* ok);
   ExpressionT ParseMemberExpressionContinuation(
       ExpressionT expression, ExpressionClassifier* classifier, bool* ok);
   ExpressionT ParseArrowFunctionLiteral(bool accept_IN,
                                         const FormalParametersT& parameters,
                                         const ExpressionClassifier& classifier,
                                         bool* ok);
@@ skipping 142 matching lines @@
       expected_property_count_(0),
       this_location_(Scanner::Location::invalid()),
       return_location_(Scanner::Location::invalid()),
       super_location_(Scanner::Location::invalid()),
       kind_(kind),
       generator_object_variable_(NULL),
       function_state_stack_(function_state_stack),
       outer_function_state_(*function_state_stack),
       scope_stack_(scope_stack),
       outer_scope_(*scope_stack),
-      return_expr_context_(ReturnExprContext::kNormal),
+      tail_call_expressions_(scope->zone()),
+      return_expr_context_(ReturnExprContext::kInsideValidBlock),
       non_patterns_to_rewrite_(0, scope->zone()),
       factory_(factory),
       next_function_is_parenthesized_(false),
       this_function_is_parenthesized_(false) {
   *scope_stack_ = scope;
   *function_state_stack = this;
   if (outer_function_state_) {
     this_function_is_parenthesized_ =
         outer_function_state_->next_function_is_parenthesized_;
     outer_function_state_->next_function_is_parenthesized_ = false;
@@ skipping 419 matching lines @@
 
 template <class Traits>
 typename ParserBase<Traits>::ExpressionT ParserBase<Traits>::ParseExpression(
     bool accept_IN, bool* ok) {
   ExpressionClassifier classifier(this);
   ExpressionT result = ParseExpression(accept_IN, &classifier, CHECK_OK);
   Traits::RewriteNonPattern(&classifier, CHECK_OK);
   return result;
 }
 
-
 template <class Traits>
 typename ParserBase<Traits>::ExpressionT ParserBase<Traits>::ParseExpression(
     bool accept_IN, ExpressionClassifier* classifier, bool* ok) {
   // Expression ::
   //   AssignmentExpression
   //   Expression ',' AssignmentExpression
 
   ExpressionClassifier binding_classifier(this);
   ExpressionT result =
       this->ParseAssignmentExpression(accept_IN, &binding_classifier, CHECK_OK);
   classifier->Accumulate(&binding_classifier,
                          ExpressionClassifier::AllProductions);
   bool is_simple_parameter_list = this->IsIdentifier(result);
   bool seen_rest = false;
   while (peek() == Token::COMMA) {
+    ValidateTailCallExpression(classifier, CHECK_OK);
     if (seen_rest) {
       // At this point the production can't possibly be valid, but we don't know
       // which error to signal.
       classifier->RecordArrowFormalParametersError(
           scanner()->peek_location(), MessageTemplate::kParamAfterRest);
     }
     Consume(Token::COMMA);
     bool is_rest = false;
     if (peek() == Token::ELLIPSIS) {
       // 'x, y, ...z' in CoverParenthesizedExpressionAndArrowParameterList only
@@ skipping 471 matching lines @@
 template <class Traits>
 typename ParserBase<Traits>::ExpressionT
 ParserBase<Traits>::ParseAssignmentExpression(bool accept_IN,
                                               ExpressionClassifier* classifier,
                                               bool* ok) {
   // AssignmentExpression ::
   //   ConditionalExpression
   //   ArrowFunction
   //   YieldExpression
   //   LeftHandSideExpression AssignmentOperator AssignmentExpression
-  //   TailCallExpression
   bool is_destructuring_assignment = false;
   int lhs_beg_pos = peek_position();
 
   if (peek() == Token::YIELD && is_generator()) {
     return this->ParseYieldExpression(accept_IN, classifier, ok);
   }
 
   FuncNameInferrer::State fni_state(fni_);
   ParserBase<Traits>::Checkpoint checkpoint(this);
   ExpressionClassifier arrow_formals_classifier(this,
@@ skipping 180 matching lines @@
   }
 
   expression = Traits::BuildIteratorResult(expression, false);
   // Hackily disambiguate o from o.next and o [Symbol.iterator]().
   // TODO(verwaest): Come up with a better solution.
   typename Traits::Type::YieldExpression yield =
       factory()->NewYield(generator_object, expression, pos);
   return yield;
 }
 
+template <class Traits>
+typename ParserBase<Traits>::ExpressionT
+ParserBase<Traits>::ParseTailCallExpression(ExpressionClassifier* classifier,
+                                            bool* ok) {
+  // TailCallExpression::
+  //   'continue' MemberExpression  Arguments
+  //   'continue' CallExpression  Arguments
+  //   'continue' MemberExpression  TemplateLiteral
+  //   'continue' CallExpression  TemplateLiteral
+  Expect(Token::CONTINUE, CHECK_OK);
+  int pos = position();
+  int sub_expression_pos = peek_position();
+  ExpressionT expression =
+      this->ParseLeftHandSideExpression(kDontAcceptTCE, classifier, ok);
+  Scanner::Location loc(pos, scanner()->location().end_pos);
+  ReturnExprContext return_expr_context =
+      function_state_->return_expr_context();
+  if (return_expr_context != ReturnExprContext::kInsideValidReturnStatement) {
+    MessageTemplate::Template msg = MessageTemplate::kNone;
+    switch (return_expr_context) {
+      case ReturnExprContext::kInsideValidReturnStatement:
+        UNREACHABLE();
+        return Traits::EmptyExpression();
+      case ReturnExprContext::kInsideValidBlock:
+        msg = MessageTemplate::kUnexpectedTailCall;
+        break;
+      case ReturnExprContext::kInsideTryBlock:
+        msg = MessageTemplate::kUnexpectedTailCallInTryBlock;
+        break;
+      case ReturnExprContext::kInsideForInOfBody:
+        msg = MessageTemplate::kUnexpectedTailCallInForInOf;
+        break;
+    }
+    ReportMessageAt(loc, msg);
+    *ok = false;
+    return Traits::EmptyExpression();
+  }
+  if (!expression->IsCall()) {
+    Scanner::Location sub_loc(sub_expression_pos, loc.end_pos);
+    ReportMessageAt(sub_loc, MessageTemplate::kUnexpectedInsideTailCall);
+    *ok = false;
+    return Traits::EmptyExpression();
+  }
+  classifier->RecordTailCallExpressionError(
+      loc, MessageTemplate::kUnexpectedTailCall);
+  function_state_->AddExpressionInTailPosition(expression, loc);
+  return expression;
+}
 
 // Precedence = 3
 template <class Traits>
 typename ParserBase<Traits>::ExpressionT
 ParserBase<Traits>::ParseConditionalExpression(bool accept_IN,
                                                ExpressionClassifier* classifier,
                                                bool* ok) {
   // ConditionalExpression ::
   //   LogicalOrExpression
   //   LogicalOrExpression '?' AssignmentExpression ':' AssignmentExpression
@@ skipping 24 matching lines @@
 template <class Traits>
 typename ParserBase<Traits>::ExpressionT
 ParserBase<Traits>::ParseBinaryExpression(int prec, bool accept_IN,
                                           ExpressionClassifier* classifier,
                                           bool* ok) {
   DCHECK(prec >= 4);
   ExpressionT x = this->ParseUnaryExpression(classifier, CHECK_OK);
   for (int prec1 = Precedence(peek(), accept_IN); prec1 >= prec; prec1--) {
     // prec1 >= 4
     while (Precedence(peek(), accept_IN) == prec1) {
+      ValidateTailCallExpression(classifier, CHECK_OK);

[rossberg, 2016/05/02 10:59:40]

Ah, okay, think I get it now: this is validating that it is _not_ a tial call
expression. Maybe you want to change the name.

[Igor Sheludko, 2016/05/04 10:28:55]

Done.

       Traits::RewriteNonPattern(classifier, CHECK_OK);
       BindingPatternUnexpectedToken(classifier);
       ArrowFormalParametersUnexpectedToken(classifier);
       Token::Value op = Next();
       int pos = position();
 
       const bool is_right_associative = op == Token::EXP;
       const int next_prec = is_right_associative ? prec1 : prec1 + 1;
       ExpressionT y =
           ParseBinaryExpression(next_prec, accept_IN, classifier, CHECK_OK);
+      if (op != Token::OR && op != Token::AND) {
+        ValidateTailCallExpression(classifier, CHECK_OK);
+      }
       Traits::RewriteNonPattern(classifier, CHECK_OK);
 
       if (this->ShortcutNumericLiteralBinaryExpression(&x, y, op, pos,
                                                        factory())) {
         continue;
       }
 
       // For now we distinguish between comparisons and other binary
       // operations.  (We could combine the two and get rid of this
       // code and AST node eventually.)
@@ skipping 95 matching lines @@
 
 template <class Traits>
 typename ParserBase<Traits>::ExpressionT
 ParserBase<Traits>::ParsePostfixExpression(ExpressionClassifier* classifier,
                                            bool* ok) {
   // PostfixExpression ::
   //   LeftHandSideExpression ('++' | '--')?
 
   int lhs_beg_pos = peek_position();
   ExpressionT expression =
-      this->ParseLeftHandSideExpression(classifier, CHECK_OK);
+      this->ParseLeftHandSideExpression(kAcceptTCE, classifier, CHECK_OK);
   if (!scanner()->HasAnyLineTerminatorBeforeNext() &&
       Token::IsCountOp(peek())) {
     BindingPatternUnexpectedToken(classifier);
     ArrowFormalParametersUnexpectedToken(classifier);
 
     expression = this->CheckAndRewriteReferenceExpression(
         expression, lhs_beg_pos, scanner()->location().end_pos,
         MessageTemplate::kInvalidLhsInPostfixOp, CHECK_OK);
     expression = this->MarkExpressionAsAssigned(expression);
     Traits::RewriteNonPattern(classifier, CHECK_OK);
 
     Token::Value next = Next();
     expression =
         factory()->NewCountOperation(next,
                                      false /* postfix */,
                                      expression,
                                      position());
   }
   return expression;
 }
 
-
 template <class Traits>
 typename ParserBase<Traits>::ExpressionT
 ParserBase<Traits>::ParseLeftHandSideExpression(
-    ExpressionClassifier* classifier, bool* ok) {
+    AcceptTailCallExpression accept_TCE, ExpressionClassifier* classifier,
+    bool* ok) {
   // LeftHandSideExpression ::
   //   (NewExpression | MemberExpression) ...
 
+  if (FLAG_harmony_explicit_tailcalls && accept_TCE == kAcceptTCE &&
+      peek() == Token::CONTINUE) {
+    return this->ParseTailCallExpression(classifier, ok);
+  }
+
   ExpressionT result =
       this->ParseMemberWithNewPrefixesExpression(classifier, CHECK_OK);
 
   while (true) {
     switch (peek()) {
       case Token::LBRACK: {
+        ValidateTailCallExpression(classifier, CHECK_OK);
         Traits::RewriteNonPattern(classifier, CHECK_OK);
         BindingPatternUnexpectedToken(classifier);
         ArrowFormalParametersUnexpectedToken(classifier);
         Consume(Token::LBRACK);
         int pos = position();
         ExpressionT index = ParseExpression(true, classifier, CHECK_OK);
         Traits::RewriteNonPattern(classifier, CHECK_OK);
         result = factory()->NewProperty(result, index, pos);
         Expect(Token::RBRACK, CHECK_OK);
         break;
       }
 
       case Token::LPAREN: {
+        ValidateTailCallExpression(classifier, CHECK_OK);
         Traits::RewriteNonPattern(classifier, CHECK_OK);
         BindingPatternUnexpectedToken(classifier);
         ArrowFormalParametersUnexpectedToken(classifier);
 
         int pos;
         if (scanner()->current_token() == Token::IDENTIFIER ||
             scanner()->current_token() == Token::SUPER) {
           // For call of an identifier we want to report position of
           // the identifier as position of the call in the stack trace.
           pos = position();
@@ skipping 38 matching lines @@
           ExpressionT this_expr = this->ThisExpression(scope_, factory(), pos);
           result =
               factory()->NewAssignment(Token::INIT, this_expr, result, pos);
         }
 
         if (fni_ != NULL) fni_->RemoveLastFunction();
         break;
       }
 
       case Token::PERIOD: {
+        ValidateTailCallExpression(classifier, CHECK_OK);
         Traits::RewriteNonPattern(classifier, CHECK_OK);
         BindingPatternUnexpectedToken(classifier);
         ArrowFormalParametersUnexpectedToken(classifier);
         Consume(Token::PERIOD);
         int pos = position();
         IdentifierT name = ParseIdentifierName(CHECK_OK);
         result = factory()->NewProperty(
             result, factory()->NewStringLiteral(name, pos), pos);
         if (fni_ != NULL) this->PushLiteralName(fni_, name);
         break;
       }
 
       case Token::TEMPLATE_SPAN:
       case Token::TEMPLATE_TAIL: {
+        ValidateTailCallExpression(classifier, CHECK_OK);
         Traits::RewriteNonPattern(classifier, CHECK_OK);
         BindingPatternUnexpectedToken(classifier);
         ArrowFormalParametersUnexpectedToken(classifier);
         result = ParseTemplateLiteral(result, position(), classifier, CHECK_OK);
         break;
       }
 
       default:
         return result;
     }
@@ skipping 457 matching lines @@
             this->EmptyIdentifier(), RelocInfo::kNoPosition, formal_parameters,
             kArrowFunction, FunctionLiteral::kAnonymousExpression, CHECK_OK);
         materialized_literal_count =
             function_state.materialized_literal_count();
         expected_property_count = function_state.expected_property_count();
       }
     } else {
       // Single-expression body
       int pos = position();
       ExpressionClassifier classifier(this);
-      bool is_tail_call_expression;
-      if (FLAG_harmony_explicit_tailcalls) {
-        // TODO(ishell): update chapter number.
-        // ES8 XX.YY.ZZ
-        if (peek() == Token::CONTINUE) {
-          Consume(Token::CONTINUE);
-          pos = position();
-          is_tail_call_expression = true;
-        } else {
-          is_tail_call_expression = false;
-        }
-      } else {
-        // ES6 14.6.1 Static Semantics: IsInTailPosition
-        is_tail_call_expression =
-            allow_tailcalls() && !is_sloppy(language_mode());
-      }
+      DCHECK(ReturnExprContext::kInsideValidBlock ==
+             function_state_->return_expr_context());
+      ReturnExprScope allow_tail_calls(
+          function_state_, ReturnExprContext::kInsideValidReturnStatement);
       ExpressionT expression =
           ParseAssignmentExpression(accept_IN, &classifier, CHECK_OK);
       Traits::RewriteNonPattern(&classifier, CHECK_OK);
       body = this->NewStatementList(1, zone());
       this->AddParameterInitializationBlock(formal_parameters, body, CHECK_OK);
       body->Add(factory()->NewReturnStatement(expression, pos), zone());
       materialized_literal_count = function_state.materialized_literal_count();
       expected_property_count = function_state.expected_property_count();
-      if (is_tail_call_expression) {
+      if (allow_tailcalls() && !is_sloppy(language_mode())) {
+        // ES6 14.6.1 Static Semantics: IsInTailPosition
         this->MarkTailPosition(expression);
       }
+      this->MarkCollectedTailCallExpressions();
     }
     super_loc = function_state.super_location();
 
     formal_parameters.scope->set_end_position(scanner()->location().end_pos);
 
     // Arrow function formal parameters are parsed as StrictFormalParameterList,
     // which is not the same as "parameters of a strict function"; it only means
     // that duplicates are not allowed.  Of course, the arrow function may
     // itself be strict as well.
     const bool allow_duplicate_parameters = false;
@@ skipping 236 matching lines @@
     has_seen_constructor_ = true;
     return;
   }
 }
 
 
 }  // namespace internal
 }  // namespace v8
 
 #endif  // V8_PARSING_PARSER_BASE_H