From 4bde1f1851a13ccffe96db9227c9493cacc0dc03 Mon Sep 17 00:00:00 2001
From: Paul Berry <paulberry@google.com>
Date: Thu, 13 Feb 2020 20:23:18 +0000
Subject: [PATCH] Migration: test to make sure exact nullability isn't applied
 too much.

This is a regression test that would have caught the mistake in
https://dart-review.googlesource.com/c/sdk/+/135351 (see that CL for
additional details).

Change-Id: Ideaf83df6deae3f6688fc89cfd8b3568344c5d4d
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/135660
Reviewed-by: Mike Fairhurst <mfairhurst@google.com>
Commit-Queue: Paul Berry <paulberry@google.com>
---
 pkg/nnbd_migration/test/api_test.dart | 31 +++++++++++++++++++++++++++
 1 file changed, 31 insertions(+)

diff --git a/pkg/nnbd_migration/test/api_test.dart b/pkg/nnbd_migration/test/api_test.dart
index 8e857deb6c3..7f7d53bd33c 100644
--- a/pkg/nnbd_migration/test/api_test.dart
+++ b/pkg/nnbd_migration/test/api_test.dart
@@ -1544,6 +1544,37 @@ void h() {
     await _checkSingleFileChanges(content, expected);
   }
 
+  Future<void> test_exact_nullability_counterexample() async {
+    var content = '''
+void f(List<int> x) {
+  x.add(1);
+}
+void g() {
+  f([null]);
+}
+void h(List<int> x) {
+  f(x);
+}
+''';
+    // The `null` in `g` causes `f`'s `x` argument to have type `List<int?>`.
+    // Even though `f` calls a method that uses `List`'s type parameter
+    // contravariantly (the `add` method), that is not sufficient to cause exact
+    // nullability propagation, since value passed to `add` has a
+    // non-nullable type.  So nullability is *not* propagated back to `h`.
+    var expected = '''
+void f(List<int?> x) {
+  x.add(1);
+}
+void g() {
+  f([null]);
+}
+void h(List<int> x) {
+  f(x);
+}
+''';
+    await _checkSingleFileChanges(content, expected);
+  }
+
   Future<void> test_explicit_nullable_overrides_hard_edge() async {
     var content = '''
 int f(int/*?*/ i) => i + 1;