[feng19]

Surfaced by the Elixir 1.20.2 / OTP 29 upgrade — the compiler's type checker reports

Symptom

warning: the following clause will never match:

    {:ok, value} ->

because it attempts to match on the result of:

    value

which has type:

    dynamic(empty_list() or non_empty_list(term(), term()))

type warning found at:
  8 │ defmodule XxxWeb.V2.XxxController do
    └─ (xxx 0.1.0) lib/xxx_web/controllers/v2/xxx_controller.ex:8:
       XxxWeb.V2.XxxController.xxx/2

The trigger is a params declaration that chains a parser function into List, e.g.

requires :uuids, (&{:ok, List.wrap(&1)}) |> List[String]
requires :data, (&{:ok, List.wrap(&1)}) |> List do ... end