warning reports a misleading message on unused default arguments

[DaTrader]

Environment

• Elixir & Erlang/OTP versions (elixir --version): 1.10.3 / OTP 22
• Operating system: Linux Mint

Current behavior

A private function taking n arguments, say 5 with the last two set to default values, when compiled without another function in the module ever invoking it with less than 4 arguments, gives a misleading warning message:
warning: the first 4 default arguments in foo/5 are never used
ex:

defp foo( arg1, arg2, arg3, arg4 \\ nil, arg5 \\ nil) do
..
end

Expected behavior

The warning message should read something like:
warning: the default value for argument no. 4 in foo/5 is never used

[josevalim]

Closing in favor of PR.

[xxdavid]

Hi! I'm following up on this issue. The messages were indeed reworded in #10097 by @kelvinst but I am still not convinced that they are correct.

Given this module

defmodule FooBar do
  defp foo(arg1, arg2, arg3, arg4 \\ nil, arg5 \\ nil) do
    {arg1, arg2, arg3, arg4, arg5}
  end

  def bar do
    foo(:a, :b, :c, :d)
  end
end

the Elixir compiler (1.15.0) reports the following:

warning: the default values for the first 4 optional arguments in foo/5 are never used
  foo.ex:2: FooBar (module)

But the message doesn't make much sense to me… It talks about first 4 optional arguments of foo/5 but the function has only 2 optional arguments! I think it wants to convey the idea that the default value of the 4th argument wasn't used (also the implementation of Module.LocalsTracker testifies this if I understand it correctly). Something like @DaTrader proposed (the default value for argument no. 4 in foo/5 is never used) or for example the default value for the 4th argument in foo/5 is never used would be more correct IMHO. What do you think?