False Positive warning about missing key in map

[Awlexus]

Elixir and Erlang/OTP versions

Erlang/OTP 27 [erts-15.2.3] [source] [64-bit] [smp:12:12] [ds:12:12:10] [async-threads:1] [jit]

Elixir 1.18.3 (compiled with Erlang/OTP 27)

Also reproducable with 1.18.4 and 1.19.0-rc.0 (OTP 27)

Operating system

MacOS 15.5

Current behavior

Trying to compile this module creates a compile-time warning

defmodule FalsePositiveTypeWarning do
  def test_val() do
    data = %{
      list: [
        %{
          one: %{}
        },
        %{
          two: %{
            three: 1
          }
        }
      ]
    }

    [_a, b] = data.list
    c = b.two
    c.three
  end
end

warning: unknown key .two in expression:

    b.two

the given type does not have the given key:

    %{one: empty_map()} or %{two: %{three: integer()}}

where "b" was given the type:

    # type: %{one: empty_map()} or %{two: %{three: integer()}}
    # from: iex:16
    [_a, b] = data.list

└─ iex:17: FalsePositiveTypeWarning.test_val/0

Expected behavior

The given code should compile without warning

[josevalim]

This warning, in particular, won't be addressed and it is one of the "false positives" that will become part of our workflows as we move forward. This is because lists are always treated as heterogeneous entities and not tracked by position. You can use tuples or maps, which are data types designed for that which allow you precise matching. Thanks for the report!

[Awlexus]

Understandable. Thankfully this only happened in a data-structure I wrote for a test. I'll try to work around it