Reland of [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 9375bb0df5bb2f04a65e3dd336829e9c573029be..e1e3d3412281a274f2a3d03079735cc7c113a2de 100644
--- a/src/parsing/parser-base.h
+++ b/src/parsing/parser-base.h
@@ -1346,6 +1346,24 @@
   bool IsValidPattern(ExpressionT expression) {
     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 initializing "declaration"
+  // does not 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,
@@ -5698,8 +5716,12 @@
   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(
@@ -5751,6 +5773,8 @@
   }
 }
 
+#undef CHECK_OK
+#undef CHECK_OK_CUSTOM
 #undef CHECK_OK_VOID
 
 }  // namespace internal