Inline in empty list in erlang pass

[sabiwara]

• x in [] is expanded at :lists.member(x, [])
• then later the erlang pass inlines it as false

Same approach as 34a5000

Closes #15411

[sabiwara]

It feels we should be able to narrow it to false (in which case we'd still have a warning, just more helpful), but right now it's consistent with :foo in [:bar] not warning.

[josevalim]

On apply.ex, you can replace this:

{:lists, :member, [{[term(), list(term())], boolean()}]},

with this:

{:lists, :member, [{[term(), empty_list()], atom([false])}, {[term(), non_empty_list(term())], boolean()}]},

[josevalim]

We need to execute the expression in case of side-effects, so it has to be something like before:

Suggested change

	{{atom, Ann, false}, S};
	{{block, Ann, [{match, Ann, {var, Ann, '_'}, Expr}, {atom, Ann, false}]}, S};

Shall we add a test to the test/erlang/* files? I think we test other optimizations there already.

[josevalim]

Or even better, perhaps a test for Kernel.in/2 like this:

false = send(self(), :hello) in []
assert_received :hello

[josevalim]

I dropped some comments.

We also need to replace this in types/helpers.ex:

      {{:., _, [:lists, :member]}, meta, [expr, [_ | _] = args]} = call ->
        if Enum.any?(args, &match?({:|, _, [_, _]}, &1)) do
          call
        else
          {:in, meta, [expr, args]}
        end

by this:

      {{:., _, [:lists, :member]}, meta, [expr, args]} = call when is_list(args) ->
        if Enum.any?(args, &match?({:|, _, [_, _]}, &1)) do
          call
        else
          {:in, meta, [expr, args]}
        end

You can add some tests to types/helper_test.exs for in (currently no coverage).

[sabiwara]

Thanks for all the pointers @josevalim 💜

OK, with 58a96ee we're back with a warning but a readable one: