# 11. Redundant defensive guarding check

meta:
  id: pygienium-11
  feature: pygienium
  priority: P2
  depends_on: [pygienium-06]
  tags: [check]

objective:

- Implement the redundant-defensive-guarding check: remove excessive null checks, unnecessary try/catch, fallback paths that mask bugs, defensive code guarding invariants the type system already guarantees.

deliverables:

- `src/checks/defensive-guards.ts`: `CheckDefinition` with scan + fix tasks
- Rubric: redundant null/undefined checks where types are non-nullable, try/catch that only rethrows or swallows, fallback values that hide errors, defensive guards on already-validated input, compatibility fallbacks (engineering rule: remove, don't layer)
- `/pygienium-defensive-guards` runs E2E

steps:

- Author `buildScanTask`: agent identifies defensive smells, writes `pygienium/checks/defensive-guards/findings.md`
- Author `buildFixTask`: remove redundant guards; preserve guards that protect real external boundaries (user input, IO, parsing); write changes.md distinguishing removed vs kept-with-reason
- Register the check

tests:

- Integration: temp file with a null check on a typed-non-null param + a try/catch that swallows; run `--fix`; assert removed; assert a JSON.parse guard is preserved

acceptance_criteria:

- findings.md separates redundant guards from legitimate boundary guards
- `--fix` removes redundant guards and keeps boundary guards (IO, parsing, untrusted input)

validation:

- Inspect `pygienium/checks/defensive-guards/{findings.md,changes.md}`

notes:

- Key judgment: guarding external boundaries (IO, untrusted input, parsing) is correct; guarding internal invariants the type system guarantees is noise
- Engineering rule: no compatibility layers or fallbacks meant to be replaced later