Credence.Pattern.NoRedundantNegatedGuard (credence v0.4.3)

Detects guard clauses that are logically redundant because a preceding clause of the same function already handles the complementary case.

Why this matters

LLMs add "safety" guards because they don't trust Elixir's clause ordering. When a clause with when a == b precedes one with when a != b, the second guard is guaranteed to pass — anything reaching that clause already failed the equality check:

# Flagged — second guard is redundant
defp compare([h1 | t1], [h2 | t2]) when h1 == h2, do: compare(t1, t2)
defp compare([h1 | _], [h2 |_ ]) when h1 != h2, do: h1

# Idiomatic — clause ordering handles it
defp compare([h1 | t1], [h1 | t2]), do: compare(t1, t2)
defp compare([missing | _],_ ), do: missing

Fix

The fix removes the redundant negated guard from the second clause. Since the preceding clause already matches the equality case, the negated guard adds no value — the clause ordering already ensures that only unequal pairs reach the second clause.

Flagged patterns

A function clause whose guard is when a != b or when a !== b, immediately preceded by a clause of the same function/arity with when a == b or when a === b (same variables, same positions).