fix(core): Fn::Transform excluded in resource deep merge

The intrinsic function `Fn::Transform` can be used anywhere in a CloudFormation
template which makes it special compared to the other intrinsic functions. In
order for CfnResources to have Fn::Transform as a property key, this intrinsic
needs to be merged like any other property.

#36203

Author: rikirkamazon

packages/aws-cdk-lib/core/lib/cfn-resource.ts

@@ -625,7 +625,12 @@ export interface ICfnResourceOptions {
 
 /**
  * Object keys that deepMerge should not consider. Currently these include
- * CloudFormation intrinsics
+ * CloudFormation intrinsics, exluding Fn::Transform.
+ *
+ * Intrinsics are objects in CloudFormation templates where the one and only key is the function name and the value is
+ * the function's arguments. Fn::Transform is a unique intrinsic as it can appear as a key in multi-key objects (like
+ * resource Properties). For this reason it is left out of this list as it should not get rid of other keys when doing object
+ * deep merges.
  *
  * @see https://docs.aws.amazon.com/AWSCloudFormation/latest/UserGuide/intrinsic-function-reference.html
  */
@@ -642,7 +647,6 @@ const MERGE_EXCLUDE_KEYS: string[] = [
   'Fn::Select',
   'Fn::Split',
   'Fn::Sub',
-  'Fn::Transform',
   'Fn::And',
   'Fn::Equals',
   'Fn::If',

packages/aws-cdk-lib/core/test/cfn-resource.test.ts

@@ -45,6 +45,60 @@ describe('cfn resource', () => {
         },
       });
     });
+
+    test('renders "Properties" for resource with Fn::Transform as an override', () => {
+      const app = new core.App();
+      const stack = new core.Stack(app, 'TestStack');
+      const resource = new core.CfnResource(stack, 'Resource', {
+        type: 'Test::Resource::Fake',
+        properties: {
+          FakeProperty: 'Foo',
+        },
+      });
+
+      resource.addOverride('Properties.Fn::Transform', {
+        Name: 'FakeTransform',
+      });
+
+      expect(app.synth().getStackByName(stack.stackName).template?.Resources).toEqual({
+        Resource: {
+          Type: 'Test::Resource::Fake',
+          Properties: {
+            'FakeProperty': 'Foo',
+            'Fn::Transform': {
+              Name: 'FakeTransform',
+            },
+          },
+        },
+      });
+    });
+
+    test('renders "Properties" for resource with Fn::Transform as a property', () => {
+      const app = new core.App();
+      const stack = new core.Stack(app, 'TestStack');
+      const resource = new core.CfnResource(stack, 'Resource', {
+        type: 'Test::Resource::Fake',
+        properties: {
+          'Fn::Transform': {
+            Name: 'FakeTransform',
+          },
+        },
+      });
+
+      resource.addOverride('Properties.FakeProperty', 'Foo');
+
+      expect(app.synth().getStackByName(stack.stackName).template?.Resources).toEqual({
+        Resource: {
+          Type: 'Test::Resource::Fake',
+          Properties: {
+            'FakeProperty': 'Foo',
+            'Fn::Transform': {
+              Name: 'FakeTransform',
+            },
+          },
+        },
+      });
+    });
   });
 
   describe('snapshot removal policy', () => {