Migration: Add the ability to use null for nullability nodes in substitutions.

stereotype441 · commit-bot@chromium.org · commit 32e27f68dc45 · 2019-08-12T19:05:00.000Z
A nullability node of `null` will be discarded during a substitution. For instance, attempting to substitute `T=int?null` into `T?1` will produce `int?1`, rather than `int?2` where `2` is a substitution node. We will need this functionality in order to compare function types for equality, which is in turn needed to integrate with flow analysis. Change-Id: Ife5e3761b083d4584b2aff760dcb104529e7f9d4 Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/112745 Reviewed-by: Konstantin Shcheglov <scheglov@google.com> Commit-Queue: Paul Berry <paulberry@google.com>
diff --git a/pkg/nnbd_migration/lib/src/nullability_node.dart b/pkg/nnbd_migration/lib/src/nullability_node.dart
@@ -410,15 +410,14 @@ abstract class NullabilityNode {
   /// variable being eliminated by the substitution, and [innerNode] is the
   /// nullability node for the type being substituted in its place.
   ///
-  /// [innerNode] may be `null`.  TODO(paulberry): when?
-  ///
-  /// Additional constraints are recorded in [constraints] as necessary to make
-  /// the new nullability node behave consistently with the old nodes.
-  /// TODO(paulberry): this should become unnecessary once constraint solving is
-  /// performed directly using [NullabilityNode] objects.
+  /// If either [innerNode] or [outerNode] is `null`, then the other node is
+  /// returned.
   factory NullabilityNode.forSubstitution(
-          NullabilityNode innerNode, NullabilityNode outerNode) =
-      NullabilityNodeForSubstitution._;
+      NullabilityNode innerNode, NullabilityNode outerNode) {
+    if (innerNode == null) return outerNode;
+    if (outerNode == null) return innerNode;
+    return NullabilityNodeForSubstitution._(innerNode, outerNode);
+  }
 
   /// Creates a [NullabilityNode] representing the nullability of a type
   /// annotation appearing explicitly in the user's program.
diff --git a/pkg/nnbd_migration/test/nullability_node_test.dart b/pkg/nnbd_migration/test/nullability_node_test.dart
@@ -594,6 +594,12 @@ class NullabilityNodeTest {
     expect(edge.isSatisfied, true);
   }
 
+  test_substitution_simplify_null() {
+    var n1 = newNode(1);
+    expect(subst(null, n1), same(n1));
+    expect(subst(n1, null), same(n1));
+  }
+
   test_unconstrainted_node_non_nullable() {
     var n1 = newNode(1);
     propagate();
