I came across this issue when using a behaviour from a dependency and did not see any warnings when I implemented a callback with the wrong number of arguments. I used mix run --no-mix-exs script.exs. Reproducible on 1.15.7 but works as expected on 1.13.4.

Here’s an MRE:

defmodule MyBehaviour do
  @callback my_fun(arg :: any) :: any
end

defmodule MyModule do
  @behaviour MyBehaviour

  @impl true
  def my_fub(arg) do
    arg
  end
end

IO.puts("Hello world")

Output using 1.13.4 and 1.15.7:

root@5da670a83b2e:/workspace/my_project# mix run --no-mix-exs script.exs 
warning: got "@impl true" for function my_fub/1 but no behaviour specifies such callback. The known callbacks are:

  * MyBehaviour.my_fun/1 (function)

  script.exs:9: MyModule (module)

warning: function my_fun/1 required by behaviour MyBehaviour is not implemented (in module MyModule)
  script.exs:5: MyModule (module)

Hello world
root@5da670a83b2e:/workspace/my_project# 

With receive block:

defmodule MyBehaviour do
  @callback my_fun(arg :: any) :: any
end

defmodule MyModule do
  @behaviour MyBehaviour

  @impl true
  def my_fub(arg) do
    arg
  end
end

IO.puts("Hello world")

receive do
  :done -> MyModule.my_fub("Hello again")
end

Output using both versions:

root@5da670a83b2e:/workspace/my_project# asdf global elixir 1.13
root@5da670a83b2e:/workspace/my_project# mix run --no-mix-exs script.exs 
warning: got "@impl true" for function my_fub/1 but no behaviour specifies such callback. The known callbacks are:

  * MyBehaviour.my_fun/1 (function)

  script.exs:9: MyModule (module)

warning: function my_fun/1 required by behaviour MyBehaviour is not implemented (in module MyModule)
  script.exs:5: MyModule (module)

Hello world
^C
BREAK: (a)bort (A)bort with dump (c)ontinue (p)roc info (i)nfo
       (l)oaded (v)ersion (k)ill (D)b-tables (d)istribution
a
root@5da670a83b2e:/workspace/my_project# asdf global elixir 1.15.7
root@5da670a83b2e:/workspace/my_project# mix run --no-mix-exs script.exs 
Hello world

I’d be grateful for any help, cheers