Fix exception in migration of ?? operator when the right hand side is a generic type

Change-Id: Ie372522f9493e2ccea8284f8583b0f43153adbbd
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/123884
Reviewed-by: Paul Berry <paulberry@google.com>
Commit-Queue: Brian Wilkerson <brianwilkerson@google.com>

Author: bwilkerson

pkg/nnbd_migration/lib/src/edge_builder.dart

@@ -378,7 +378,9 @@ class EdgeBuilder extends GeneralizingAstVisitor<DecoratedType>
           rightType = rightOperand.accept(this);
         });
         var ifNullNode = NullabilityNode.forIfNotNull();
-        expressionType = DecoratedType(node.staticType, ifNullNode);
+        expressionType = _decorateUpperOrLowerBound(
+            node, node.staticType, leftType, rightType, true,
+            node: ifNullNode);
         _connect(
             rightType.node, expressionType.node, IfNullOrigin(source, node));
       } finally {

pkg/nnbd_migration/test/edge_builder_test.dart

@@ -1201,6 +1201,15 @@ int f(int i, int j) => i ?? j;
     assertEdge(right, expression, guards: [left], hard: false);
   }
 
+  test_binaryExpression_questionQuestion_genericReturnType() async {
+    await analyze('''
+class C<E> {
+  C<E> operator +(C<E> c) => this;
+}
+C<int> f(C<int> i, C<int> j) => i ?? j;
+''');
+  }
+
   test_binaryExpression_right_dynamic() async {
     await analyze('''
 class C {