[parsing] Improve elision of hole checks for default parameters

Use the position of the next parameter to be declared as the end of the
initializer for default parameters, so that hole checks can be elided
for initializers using previous parameters in arrow functions.

This fixes a source of bytecode mismatches when collecting source
positions lazily.

Bug: chromium:980422, v8:8510
Change-Id: I5ab074231248b661156e7d8e47c01685448b56d5
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/1683267Reviewed-by: Toon Verwaest <verwaest@chromium.org>
Commit-Queue: Dan Elphick <delphick@chromium.org>
Cr-Commit-Position: refs/heads/master@{#62525}

src/parsing/expression-scope.h

@@ -695,9 +695,15 @@ class ArrowHeadParsingScope : public ExpressionParsingScope<Types> {
       }
     }
 
-    int initializer_position = this->parser()->end_position();
+    auto var_it = this->variable_list()->begin();
     for (auto declaration : *result->declarations()) {
-      declaration->var()->set_initializer_position(initializer_position);
+      // If it's not the last variable, then use the position of the next
+      // variable. For the last one, use the end position of the arrow head.
+      int end_position = var_it + 1 == this->variable_list()->end()
+                             ? this->parser()->end_position()
+                             : (*(var_it + 1))->position();
+      declaration->var()->set_initializer_position(end_position);
+      var_it++;
     }
     if (uses_this_) result->UsesThis();
     return result;

test/mjsunit/regress/regress-crbug-980422.js

+// Copyright 2019 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.
+
+// Flags: --enable-lazy-source-positions
+try {
+  (function () {
+    ((d, e = d) => {
+      throw new Error();
+    })();
+  })();
+} catch (ex) {
+  print(ex.stack);
+}
+
+try {
+  (function () {
+    ((d, e = f, f = d) => {
+      // Won't get here as the initializers will cause a ReferenceError
+    })();
+  })();
+} catch (ex) {
+  print(ex.stack);
+}