Static Type warning on is_struct(hd(list)) guard

[tjchambers]

Elixir and Erlang/OTP versions

Elixir 1.20.0-rc.3 (9b80ab5) (compiled with Erlang/OTP 28)
Erlang OTP 28

Operating system

MacOs 26.3

Current behavior

In rc.3 I am getting a new type checking warning for functions I have guards on lists to ensure the list contains terms of the proper type.

Example:

def filter(tags, value)
      when is_list(tags) and is_struct(hd(tags)) and is_binary(value) do
    converted_value = to_regex(value)
Enum.filter(tags, fn tag -> tag.tag =~ ~r|^#{converted_value}|i end)
end

warning: incompatible types given to Kernel.is_map_key/2:

    is_map_key(hd(tags), :__struct__)

given types:

    term(), :__struct__

but expected one of:

    #1
    %{..., __struct__: term()}, term()

    #2
    %{..., __struct__: not_set()}, term()

where "tags" was given the types:

    # type: empty_list() or non_empty_list(term(), term())
    # from: lib/sct/joe/services/tag_matcher.ex:27:12
    is_list(tags)

    # type: non_empty_list(term(), term())
    # from: lib/sct/joe/services/tag_matcher.ex:27:40
    hd(tags)

type warning found at:
│

│ when is_list(tags) and is_struct(hd(tags)) and is_binary(value) do

My guard for tags is attempting to ensure that the tags are a list, and that list is non-empty and that it contains structs. I have not specified that type of the structure since there are a number of compatible structs that this function serves.

I do not quite understand why this should result in a type check warning. Any guidance would be useful. I have dozens of similarly constructed function guards which are all emitting this type of warning.

Expected behavior

No type checking warning