Add coverage for issue 303 branches

[RasputinKaiser]

Summary

• add focused coverage for issue Improve test coverage: untested branches #303's pointer/WithoutDereference branch
• add transformer-error propagation coverage through struct-field merging
• cover several Map argument validation paths and nested map-to-struct recursion
• cover the empty-string guard in changeInitialCase

Duplicate / scope check

• Addresses Improve test coverage: untested branches #303
• Checked Improve test coverage: untested branches #303 comments/assignees; no assignee or work claim was present
• gh pr list --repo darccio/mergo --state all --search "303 Improve test coverage untested branches" returned no matching PRs
• Scoped to tests only; no production behavior changes

Verification

• mise exec go@1.25.10 -- go test ./...
• mise exec go@1.25.10 -- go test -cover ./...
• git diff --check

Summary by CodeRabbit

• Tests
  • Added regression tests covering error handling, edge cases, and behavior validation for merge and map operations.

[coderabbitai]

No actionable comments were generated in the recent review. 🎉

ℹ️ Recent review info

⚙️ Run configuration

Configuration used: defaults

Review profile: CHILL

Plan: Pro

Run ID: 6c484038-8f0b-4651-b137-4c069dfdb213

📥 Commits

Reviewing files that changed from the base of the PR and between bd27904 and 1e1c7e9.

📒 Files selected for processing (1)

• issue303_test.go

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📝 Walkthrough

Walkthrough

This PR adds a new regression test file issue303_test.go to the mergo package containing five test cases that validate error handling during merge operations, map validation, and recursive structure mapping. A custom transformer helper produces errors on demand to verify proper error propagation through the merge and map APIs.

Changes

Issue 303 regression tests

Layer / File(s)	Summary
Test infrastructure and transformer helper issue303_test.go	Package setup with a custom issue303Transformer type implementing the Transformer interface to return configurable errors for non-int types, enabling downstream tests to validate error propagation.
Regression test cases issue303_test.go	Five test functions verify: isEmptyValue treats non-nil pointers as non-empty when dereferencing is disabled; Merge propagates transformer errors from struct field operations; Map returns specific sentinel errors for invalid destination/source combinations; changeInitialCase preserves empty strings; and Map recursively populates nested struct fields from nested map structures.

Estimated code review effort

🎯 2 (Simple) | ⏱️ ~10 minutes

Poem

🐰 A transformer that makes merges squeal,
Five tests to prove the error is real.
Nested maps dance through struct fields with glee,
Issue 303 resolved—now all can be free!
Regression tests hop, ensuring no fright,
When mappers and mergers run through the night. 🌙

🚥 Pre-merge checks | ✅ 4 | ❌ 1

❌ Failed checks (1 warning)

Check name	Status	Explanation	Resolution
Docstring Coverage	⚠️ Warning	Docstring coverage is 0.00% which is insufficient. The required threshold is 80.00%.	Write docstrings for the functions missing them to satisfy the coverage threshold.

✅ Passed checks (4 passed)

Check name	Status	Explanation
Description Check	✅ Passed	Check skipped - CodeRabbit’s high-level summary is enabled.
Title check	✅ Passed	The title 'Add coverage for issue 303 branches' directly and clearly describes the main change: adding test coverage for issue 303 branches, which is exactly what the changeset accomplishes.
Linked Issues check	✅ Passed	Check skipped because no linked issues were found for this pull request.
Out of Scope Changes check	✅ Passed	Check skipped because no linked issues were found for this pull request.

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✨ Finishing Touches

🧪 Generate unit tests (beta)

• Create PR with unit tests

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