[parser] parse arrow function only if no linefeed before =>

BUG=v8:3954
LOG=N
R=arv@chromium.org

Review URL: https://codereview.chromium.org/987203003

Cr-Commit-Position: refs/heads/master@{#27122}

src/preparser.h

@@ -538,6 +538,7 @@ class ParserBase : public Traits {
   }
 
   void ReportUnexpectedToken(Token::Value token);
+  void ReportUnexpectedTokenAt(Scanner::Location location, Token::Value token);
 
   // Recursive descent functions:
 
@@ -1681,7 +1682,13 @@ ParserBase<Traits>::FunctionState::~FunctionState() {
 
 template<class Traits>
 void ParserBase<Traits>::ReportUnexpectedToken(Token::Value token) {
-  Scanner::Location source_location = scanner()->location();
+  return ReportUnexpectedTokenAt(scanner_->location(), token);
+}
+
+
+template<class Traits>
+void ParserBase<Traits>::ReportUnexpectedTokenAt(
+    Scanner::Location source_location, Token::Value token) {
 
   // Four of the tokens are treated specially
   switch (token) {
@@ -2849,6 +2856,15 @@ typename ParserBase<Traits>::ExpressionT
 ParserBase<Traits>::ParseArrowFunctionLiteral(int start_pos,
                                               ExpressionT params_ast,
                                               bool* ok) {
+  if (peek() == Token::ARROW && scanner_->HasAnyLineTerminatorBeforeNext()) {
+    // ASI inserts `;` after arrow parameters if a line terminator is found.
+    // `=> ...` is never a valid expression, so report as syntax error.
+    // If next token is not `=>`, it's a syntax error anyways.
+    ReportUnexpectedTokenAt(scanner_->peek_location(), Token::ARROW);
+    *ok = false;
+    return this->EmptyExpression();
+  }
+
   Scope* scope = this->NewScope(scope_, ARROW_SCOPE);
   typename Traits::Type::StatementList body;
   int num_parameters = -1;

test/cctest/test-parsing.cc

@@ -5564,3 +5564,21 @@ TEST(StrongForIn) {
   RunParserSyncTest(strong_context_data, data, kError, NULL, 0, always_flags,
                     arraysize(always_flags));
 }
+
+
+TEST(ArrowFunctionASIErrors) {
+  const char* context_data[][2] = {{"'use strict';", ""}, {"", ""},
+                                   {NULL, NULL}};
+
+  const char* data[] = {
+      "(a\n=> a)(1)",
+      "(a/*\n*/=> a)(1)",
+      "((a)\n=> a)(1)",
+      "((a)/*\n*/=> a)(1)",
+      "((a, b)\n=> a + b)(1, 2)",
+      "((a, b)/*\n*/=> a + b)(1, 2)",
+      NULL};
+  static const ParserFlag always_flags[] = {kAllowHarmonyArrowFunctions};
+  RunParserSyncTest(context_data, data, kError, NULL, 0, always_flags,
+                    arraysize(always_flags));
+}