Hi, I am learning Elixir and recently reading the Supervisor source code, and came accross start_link/2 and start_link/3:

# https://github.com/elixir-lang/elixir/blob/v1.20.1/lib/elixir/lib/supervisor.ex#L733
def start_link(children, options) when is_list(children) do

# https://github.com/elixir-lang/elixir/blob/v1.20.1/lib/elixir/lib/supervisor.ex#L976
def start_link(module, init_arg, options \\ []) when is_list(options)

As far as I understand, the default argument in def start_link(module, init_arg, options \\ []) should cause another start_link/2 to be defined and should result in ambiguity ? Does guard helps the compiler reduce ambiguity and thus the compiler treats it as a second clause for start_link/2 ? If so, the two clauses does not need to be grouped together ?

I tried the following:

defmodule A do
  def f(a), do: "f(#{inspect(a)})"

  def f(a, b \\ []) when is_list(b), do: "f(#{inspect(a)}, #{inspect(b)})"
end

and result in a warning:

➜  /tmp iex test.ex
Erlang/OTP 27 [erts-15.2] [source] [64-bit] [smp:4:4] [ds:4:4:10] [async-threads:1] [jit:ns]

    warning: this clause for f/1 cannot match because a previous clause at line 2 always matches
    │
  4 │   def f(a, b \\ []) when is_list(b), do: "f(#{inspect(a)}, #{inspect(b)})"
    │       ~
    │
    └─ test.ex:4:7

Interactive Elixir (1.19.0-dev) - press Ctrl+C to exit (type h() ENTER for help)

But if I changed the order of the def’s:

defmodule A do
  def f(a, b \\ []) when is_list(b), do: "f(#{inspect(a)}, #{inspect(b)})"

  def f(a), do: "f(#{inspect(a)})"
end

it results in an error:

➜  /tmp iex test.ex
Erlang/OTP 27 [erts-15.2] [source] [64-bit] [smp:4:4] [ds:4:4:10] [async-threads:1] [jit:ns]

    error: def f/1 conflicts with defaults from f/2
    │
  4 │   def f(a), do: "f(#{inspect(a)})"
    │       ^
    │
    └─ test.ex:4:7: A.f/1

** (CompileError) test.ex: cannot compile module A (errors have been logged)
    test.ex:4: (module)

Can anybody point me to related documentation about this behavior difference ?