From e6cbfd71f1b3dc41a1eb414c80f6478a5e0ee905 Mon Sep 17 00:00:00 2001
From: Alexander Markov <alexmarkov@google.com>
Date: Tue, 30 Jun 2020 17:14:14 +0000
Subject: [PATCH] [vm/corelib/nnbd] Fix null check in _GrowableList.length=
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We should verify that element type is nullable every time length is
increased, not only when capacity is increased.

Fixes https://github.com/dart-lang/sdk/issues/42502

Change-Id: Id91c702a99028634da8c2d41ae0ceac521af2cf5
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/152900
Reviewed-by: Régis Crelier <regis@google.com>
Reviewed-by: Lasse R.H. Nielsen <lrn@google.com>
Commit-Queue: Alexander Markov <alexmarkov@google.com>
---
 sdk/lib/_internal/vm/lib/growable_array.dart |  9 ++++---
 tests/corelib/regress_42502_test.dart        | 27 ++++++++++++++++++++
 2 files changed, 32 insertions(+), 4 deletions(-)
 create mode 100644 tests/corelib/regress_42502_test.dart

diff --git a/sdk/lib/_internal/vm/lib/growable_array.dart b/sdk/lib/_internal/vm/lib/growable_array.dart
index bca95abd0f4..6176cd8dae5 100644
--- a/sdk/lib/_internal/vm/lib/growable_array.dart
+++ b/sdk/lib/_internal/vm/lib/growable_array.dart
@@ -120,15 +120,16 @@ class _GrowableList<T> extends ListBase<T> {
   int get length native "GrowableList_getLength";
 
   void set length(int new_length) {
-    int old_capacity = _capacity;
-    int new_capacity = new_length;
-    if (new_capacity > old_capacity) {
+    if (new_length > length) {
       // Verify that element type is nullable.
       null as T;
-      _grow(new_capacity);
+      if (new_length > _capacity) {
+        _grow(new_length);
+      }
       _setLength(new_length);
       return;
     }
+    final int new_capacity = new_length;
     // We are shrinking. Pick the method which has fewer writes.
     // In the shrink-to-fit path, we write |new_capacity + new_length| words
     // (null init + copy).
diff --git a/tests/corelib/regress_42502_test.dart b/tests/corelib/regress_42502_test.dart
new file mode 100644
index 00000000000..c3bac6770c2
--- /dev/null
+++ b/tests/corelib/regress_42502_test.dart
@@ -0,0 +1,27 @@
+// Copyright (c) 2020, the Dart project authors.  Please see the AUTHORS file
+// for details. All rights reserved. Use of this source code is governed by a
+// BSD-style license that can be found in the LICENSE file.
+
+// Regression test for https://github.com/dart-lang/sdk/issues/42502.
+// Requirements=nnbd-strong
+
+import "package:expect/expect.dart";
+
+void main() {
+  List<int> x = [];
+  Expect.throws(() {
+    x.length = x.length + 1;
+  });
+  Expect.equals(0, x.length);
+  x.add(222);
+  Expect.equals(1, x.length);
+  Expect.throws(() {
+    x.length = 2;
+  });
+  Expect.equals(1, x.length);
+  Expect.throws(() {
+    x.length = x.length + 1;
+  });
+  Expect.equals(1, x.length);
+  Expect.equals(222, x[0]);
+}