diff --git a/pkg/nnbd_migration/lib/src/variables.dart b/pkg/nnbd_migration/lib/src/variables.dart
index acd3a687781..7d8a6a01c1f 100644
--- a/pkg/nnbd_migration/lib/src/variables.dart
+++ b/pkg/nnbd_migration/lib/src/variables.dart
@@ -4,6 +4,7 @@
 
 import 'package:analyzer/dart/ast/ast.dart';
 import 'package:analyzer/dart/element/element.dart';
+import 'package:analyzer/dart/element/type.dart';
 import 'package:analyzer/src/dart/element/handle.dart';
 import 'package:analyzer/src/dart/element/type.dart';
 import 'package:analyzer/src/generated/source.dart';
@@ -245,14 +246,18 @@ class Variables implements VariableRecorder, VariableRepository {
   /// class.
   Map<ClassElement, DecoratedType> _decorateDirectSupertypes(
       ClassElement class_) {
-    if (class_.type.isObject) {
-      // TODO(paulberry): this special case is just to get the basic
-      // infrastructure working (necessary since all classes derive from
-      // Object).  Once we have the full implementation this case shouldn't be
-      // needed.
-      return const {};
+    var result = <ClassElement, DecoratedType>{};
+    for (var supertype in class_.allSupertypes) {
+      var decoratedSupertype =
+          _alreadyMigratedCodeDecorator.decorate(supertype);
+      assert(identical(decoratedSupertype.node, _graph.never));
+      var class_ = (decoratedSupertype.type as InterfaceType).element;
+      if (class_ is ClassElementHandle) {
+        class_ = (class_ as ClassElementHandle).actualElement;
+      }
+      result[class_] = decoratedSupertype;
     }
-    throw UnimplementedError('TODO(paulberry)');
+    return result;
   }
 
   int _uniqueOffsetForTypeAnnotation(TypeAnnotation typeAnnotation) =>
diff --git a/pkg/nnbd_migration/test/edge_builder_test.dart b/pkg/nnbd_migration/test/edge_builder_test.dart
index 81465ac042e..fa146414ab8 100644
--- a/pkg/nnbd_migration/test/edge_builder_test.dart
+++ b/pkg/nnbd_migration/test/edge_builder_test.dart
@@ -410,6 +410,25 @@ class C<T extends List<int>> {
         hard: false);
   }
 
+  test_assign_upcast_generic() async {
+    await analyze('''
+void f(Iterable<int> x) {}
+void g(List<int> x) {
+  f(x);
+}
+''');
+
+    var iterableInt = decoratedTypeAnnotation('Iterable<int>');
+    var listInt = decoratedTypeAnnotation('List<int>');
+    assertEdge(listInt.node, iterableInt.node, hard: true);
+    var substitution = graph
+        .getUpstreamEdges(iterableInt.typeArguments[0].node)
+        .single
+        .primarySource as NullabilityNodeForSubstitution;
+    expect(substitution.innerNode, same(listInt.typeArguments[0].node));
+    expect(substitution.outerNode, same(never));
+  }
+
   test_assignmentExpression_field() async {
     await analyze('''
 class C {