The fact that you can see the :__struct__ key in the inspect output is meant to warn you that you no longer have a valid %Cat{} struct.

You get a similar error if you add a non-existant key to a struct directly with Map.put:

iex> Map.put(cat, :some_bad_key, "val")
%{__struct__: Cat, some_bad_key: "val"}

It’s present, that’s how inspect knows to format it as %Cat{...} instead of %{...}.

You can even write code that matches on __struct__:

def thing_that_captures_struct(%struct_var{} = input) do
  # if you pass in a %Cat{}, "input" will be bound to the atom Cat
end

Nitpick: since this has a complex expression on the left-hand side, “initialize” is not quite the right verb - as the error message says, this is a failure to match. This would be fine:

%{__struct__: Cat} = %Cat{type: "Maine Coon"}

since the match requires all the keys in the map on the left to be present in the map on the right.

Structs are only maps underneath, with some compiler guarantees. In this case, the compiler cannot infer that the deletion should not happen, so it cannot warn about it. In theory the Map.delete function could check if the given map has a :__struct__ key at runtime, but currently it doesn’t do that (and I don’t think it will be changed).

Just don’t break the structs, Elixir is a dynamic language and requires some countenance.

And in my experience they’re pretty hard to make in real world code by accident. I’ve been coding Elixir full time since structs were added to the language and I can’t think of any time this has been an issue for me.