fix: Don't assume that all service bindings existed before (#2405)

smithy-aws-traits/src/main/java/software/amazon/smithy/aws/traits/auth/diff/SigV4Migration.java

@@ -83,23 +83,27 @@ public List<ValidationEvent> evaluate(Differences differences) {
             }
             // Validate Operation effective auth schemes
             for (ServiceShape newServiceShape : newOperationServiceBindings) {
-                ServiceShape oldServiceShape = oldModel.expectShape(newServiceShape.getId(), ServiceShape.class);
-                OperationShape oldOperationShape = change.getOldShape();
-                List<ShapeId> oldAuthSchemes = oldServiceIndex
-                    .getEffectiveAuthSchemes(oldServiceShape, oldOperationShape)
-                    .keySet()
-                    .stream()
-                    .collect(Collectors.toList());
-                List<ShapeId> newAuthSchemes = newServiceIndex
-                    .getEffectiveAuthSchemes(newServiceShape, newOperationShape)
-                    .keySet()
-                    .stream()
-                    .collect(Collectors.toList());
-                validateMigration(
-                    newOperationShape,
-                    oldAuthSchemes,
-                    newAuthSchemes,
-                    events);
+                oldModel.getShape(newServiceShape.getId())
+                    .filter(Shape::isServiceShape)
+                    .map(s -> (ServiceShape) s)
+                    .ifPresent(oldServiceShape -> {
+                        OperationShape oldOperationShape = change.getOldShape();
+                        List<ShapeId> oldAuthSchemes = oldServiceIndex
+                            .getEffectiveAuthSchemes(oldServiceShape, oldOperationShape)
+                            .keySet()
+                            .stream()
+                            .collect(Collectors.toList());
+                        List<ShapeId> newAuthSchemes = newServiceIndex
+                            .getEffectiveAuthSchemes(newServiceShape, newOperationShape)
+                            .keySet()
+                            .stream()
+                            .collect(Collectors.toList());
+                        validateMigration(
+                            newOperationShape,
+                            oldAuthSchemes,
+                            newAuthSchemes,
+                            events);
+                    });
             }
         }
 

smithy-aws-traits/src/test/resources/software/amazon/smithy/aws/traits/diffs/no-old-sigv4-new-service.a.smithy

@@ -0,0 +1,21 @@
+$version: "2.0"
+
+metadata suppressions = [
+    {
+        id: "ModifiedTrait.Update.smithy.api#auth"
+        namespace: "ns.foo"
+    }
+]
+
+namespace ns.foo
+
+@auth([])
+@httpBearerAuth
+service Service {
+    operations: [
+        Operation
+    ]
+}
+
+@auth([])
+operation Operation {}

smithy-aws-traits/src/test/resources/software/amazon/smithy/aws/traits/diffs/no-old-sigv4-new-service.b.smithy

@@ -0,0 +1,30 @@
+$version: "2.0"
+
+metadata suppressions = [
+    {
+        id: "ModifiedTrait.Update.smithy.api#auth"
+        namespace: "ns.foo"
+    }
+]
+
+namespace ns.foo
+
+@auth([httpBearerAuth])
+@httpBearerAuth
+service Service {
+    operations: [
+        Operation
+    ]
+}
+
+@auth([httpBearerAuth])
+@httpBearerAuth
+service NewService {
+    operations: [
+        Operation
+    ]
+}
+
+
+@auth([httpBearerAuth])
+operation Operation {}

smithy-aws-traits/src/test/resources/software/amazon/smithy/aws/traits/diffs/no-old-sigv4-new-service.events

@@ -0,0 +1 @@
+[NOTE] ns.foo#NewService: Added service `ns.foo#NewService` | AddedShape