[parsing] Fix maybe-assigned for loop variables.

src/parsing/parser-base.h

Index: src/parsing/parser-base.h
diff --git a/src/parsing/parser-base.h b/src/parsing/parser-base.h
index 56b40abf987917ad7df9b94180daacba8597ad24..58b6e3645ab6535b7c63e1d25f946e7b87507be5 100644
--- a/src/parsing/parser-base.h
+++ b/src/parsing/parser-base.h
@@ -1347,6 +1347,24 @@ class ParserBase {
     return expression->IsObjectLiteral() || expression->IsArrayLiteral();
   }
 
+  // Due to hoisting, the value of a 'var'-declared variable may actually change
+  // even if the code contains only the "initial" assignment, namely when that
+  // assignment occurs inside a loop.  For example:
+  //
+  //   let i = 10;
+  //   do { var x = i } while (i--):
+  //
+  // As a simple and very conservative approximation of this, we explicitly mark
+  // as maybe-assigned any non-lexical variable whose "declaration" does not

[marja, 2017/02/06 21:08:43]

Confused by this comment + the comment about preparser now thinking stuff is
assigned and parser thinking not assigned.

+  // syntactically occur in the function scope.  (In the example above, it
+  // occurs in a block scope.)
+  //
+  // Note that non-lexical variables include temporaries, which may also get
+  // assigned inside a loop due to the various rewritings that the parser
+  // performs.
+  //
+  static void MarkLoopVariableAsAssigned(Scope* scope, Variable* var);
+
   // Keep track of eval() calls since they disable all local variable
   // optimizations. This checks if expression is an eval call, and if yes,
   // forwards the information to scope.
@@ -5699,8 +5717,12 @@ typename ParserBase<Impl>::StatementT ParserBase<Impl>::ParseStandardForLoop(
   return loop;
 }
 
-#undef CHECK_OK
-#undef CHECK_OK_CUSTOM
+template <typename Impl>
+void ParserBase<Impl>::MarkLoopVariableAsAssigned(Scope* scope, Variable* var) {
+  if (!IsLexicalVariableMode(var->mode()) && !scope->is_function_scope()) {
+    var->set_maybe_assigned();
+  }
+}
 
 template <typename Impl>
 void ParserBase<Impl>::ObjectLiteralChecker::CheckDuplicateProto(
@@ -5752,6 +5774,8 @@ void ParserBase<Impl>::ClassLiteralChecker::CheckClassMethodName(
   }
 }
 
+#undef CHECK_OK
+#undef CHECK_OK_CUSTOM
 #undef CHECK_OK_VOID
 
 }  // namespace internal