fix(CLI): sam resources hidden in changeset diffs

packages/@aws-cdk/cloudformation-diff/lib/diff-template.ts

@@ -221,6 +221,10 @@ function addImportInformation(diff: types.TemplateDiff, changeSet: CloudFormatio
 function filterFalsePositivies(diff: types.TemplateDiff, changeSet: CloudFormation.DescribeChangeSetOutput) {
   const replacements = findResourceReplacements(changeSet);
   diff.resources.forEachDifference((logicalId: string, change: types.ResourceDifference) => {
+    if (change.resourceType.includes('AWS::Serverless')) {
+      // CFN applies the SAM transform before creating the changeset, so the changeset contains no information about SAM resources
+      return;
+    }
     change.forEachDifference((type: 'Property' | 'Other', name: string, value: types.Difference<any> | types.PropertyDifference<any>) => {
       if (type === 'Property') {
         if (!replacements[logicalId]) {

packages/@aws-cdk/cloudformation-diff/test/diff-template.test.ts

@@ -1118,6 +1118,55 @@ describe('changeset', () => {
     expect(differences.resources.differenceCount).toBe(1);
   });
 
+  test('SAM Resources are rendered with changeset diffs', () => {
+    // GIVEN
+    const currentTemplate = {
+      Resources: {
+        ServerlessFunction: {
+          Type: 'AWS::Serverless::Function',
+          Properties: {
+            CodeUri: 's3://bermuda-triangle-1337-bucket/old-handler.zip',
+          },
+        },
+      },
+    };
+
+    // WHEN
+    const newTemplate = {
+      Resources: {
+        ServerlessFunction: {
+          Type: 'AWS::Serverless::Function',
+          Properties: {
+            CodeUri: 's3://bermuda-triangle-1337-bucket/new-handler.zip',
+          },
+        },
+      },
+    };
+
+    let differences = fullDiff(currentTemplate, newTemplate, {
+      Changes: [
+        {
+          Type: 'Resource',
+          ResourceChange: {
+            Action: 'Modify',
+            LogicalResourceId: 'ServerlessFunction',
+            ResourceType: 'AWS::Lambda::Function', // The SAM transform is applied before the changeset is created, so the changeset has a Lambda resource here!
+            Replacement: 'False',
+            Details: [{
+              Evaluation: 'Direct',
+              Target: {
+                Attribute: 'Properties',
+                Name: 'Code',
+                RequiresRecreation: 'Never',
+              },
+            }],
+          },
+        },
+      ],
+    });
+    expect(differences.resources.differenceCount).toBe(1);
+  });
+
   test('imports are respected for new stacks', async () => {
     // GIVEN
     const currentTemplate = {};