Credence.Pattern.NoDuplicateFunctionClauses (credence v0.8.0)

Detects duplicate function clauses with identical argument patterns.

In Elixir, a second clause whose argument patterns are structurally identical to an earlier clause is unreachable — the first clause always matches first. The compiler emits "this clause cannot match because a previous clause always matches" which blocks compilation under --warnings-as-errors.

Two clauses have identical patterns when their arguments normalize to the same structure: bare variables are positionally equivalent regardless of name, and literal/structural patterns must match exactly.

Bad

defmodule Example do
  def bar(x, y), do: {x, y}
  def bar(x, y), do: {x, y}
end

Good

defmodule Example do
  def bar(x, y), do: {x, y}
end