[fukunaga-kengo]

Issue # (if applicable)

Fixes #390.

Fixes an assertion failure when a CloudFormation resource's Metadata is removed.

The template/change-set merger intentionally hides resource Metadata differences. It previously
did this by constructing a replacement difference from the new Metadata value twice. When Metadata
was removed, both values were undefined, violating the Difference invariant and throwing an
AssertionError.

The fix marks the existing Metadata difference as unchanged instead of constructing another
difference. This preserves the existing behavior of hiding Metadata-only changes and handles
additions, modifications, and removals uniformly.

Metadata removed
      |
      v
existing Difference(old, undefined)
      |
      v
isDifferent = false
      |
      v
no crash; Metadata remains hidden

Testing

• Added a regression test that reproduced the AssertionError before the fix.
• @aws-cdk/cloudformation-diff: 147 tests passed.
• Package compile, lint, coverage, license check, and packaging passed.

By submitting this pull request, I confirm that my contribution is made under the terms of the Apache-2.0 license

[mrgrain]

@abhiramshaji-cyber @fukunaga-kengo Thanks for both of your fixes! I think each PR is missing a test case:

• fix(cloudformation-diff): handle removed resource metadata #1741 is missing the remove/add
• fix(cloudformation-diff): diff crashes on removed resource metadata #2106 is not testing the complete absence of the Metadata key

[codecov-commenter]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 92.08%. Comparing base (95683e3) to head (b0f9fe9).
⚠️ Report is 5 commits behind head on main.

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #1741   +/-   ##
=======================================
  Coverage   92.08%   92.08%           
=======================================
  Files          83       83           
  Lines       13246    13246           
  Branches     1886     1886           
=======================================
  Hits        12197    12197           
  Misses       1011     1011           
  Partials       38       38           

Flag	Coverage Δ
suite.unit	92.08% <ø> (ø)

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[maintainer-for-aws]

Automated review

A maintainer will still review this — treat the notes below as a starting point.

📌 Commit b0f9fe9

This PR fixes a bug in the @aws-cdk/cloudformation-diff package where diffing two CloudFormation templates threw an AssertionError when a resource's Metadata block was removed. In TemplateAndChangeSetDiffMerger, Metadata changes were previously suppressed by constructing a no-op Difference(value.newValue, value.newValue); when Metadata is removed, newValue is undefined, so both constructor arguments were undefined, which the Difference constructor rejects. The fix replaces that construction with an in-place value.isDifferent = false, which the downstream onlyChanges filter already keys off of, so Metadata is suppressed from the visible and counted diff without invoking the throwing constructor. The change is minimal and behavior-preserving, mirrors the existing mutation pattern used for hidden property differences a few lines above, breaks no exported contract, and adds a parameterized regression test covering the removed, added, set-to-undefined, and added-over-undefined cases. No issues found on this change.

🔴 0 blocking · 🟡 0 recommended · ⚪ 0 optional

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[maintainer-for-aws]

See the review summary comment for the overview; the notes below are inline.

[abhiramshaji-cyber]

Congrats @fukunaga-kengo on getting this fix in, and thanks @mrgrain for the thoughtful review and for folding the extra test cases into the table. Really glad the removed and added cases from #2106 could help round out the coverage 🙂

[fukunaga-kengo]

Thanks @mrgrain for adding the extra test cases and reviewing this PR! Thanks also to @abhiramshaji-cyber for sharing the removal and addition tests from #2106. I really appreciate the help from both of you!