Awlexus

Warning about missing pattern match, despite pattern matching

I’m getting the following warning when compiling my project and I’m not sure how to resolve it, because from what I see there is a proper pattern match

Warning

    warning: a struct for Plug.Upload is expected on struct update:

        %Plug.Upload{image | filename: random_filename(name)}

    but got type:

        dynamic()

    where "image" was given the type:

        # type: dynamic()
        # from: my_file.ex:95:79
        %{"image" => %Plug.Upload{filename: name} = image} = params

    when defining the variable "image", you must also pattern match on "%Plug.Upload{}".

    hint: given pattern matching is enough to catch typing errors, you may optionally convert the struct update into a map update. For example, instead of:

        user = some_function()
        %User{user | name: "John Doe"}

    it is enough to write:

        %User{} = user = some_function()
        %{user | name: "John Doe"}

    typing violation found at:
    │
 96 │     image = %Plug.Upload{image | filename: random_filename(name)}
    │             ~
    │
    └─ my_file.ex:96:13: MyModule.put_random_filename/1

Code

  defp put_random_filename(%{"image" => %Plug.Upload{filename: name} = image} = params) do
    image = %Plug.Upload{image | filename: random_filename(name)}
    %{params | "image" => image}
  end

  defp put_random_filename(params), do: params

Elixir -v

Erlang/OTP 28 [erts-16.1.1] [source] [64-bit] [smp:12:12] [ds:12:12:10] [async-threads:1] [jit]

Elixir 1.19.2 (compiled with Erlang/OTP 28)

Different variations that trigger the same warning

No match against file name

  defp put_random_filename(%{"image" => %Plug.Upload{} = image} = params) do
    image = %Plug.Upload{image | filename: random_filename(image.filename)}
    %{params | "image" => image}
  end

Flipping the assignment

  defp put_random_filename(%{"image" => image = %Plug.Upload{filename: name}} = params) do
    image = %Plug.Upload{image | filename: random_filename(name)}
    %{params | "image" => image}
  end

11 comments

#type-checking

2 435 11

2025-11-15 23:13:53 UTC

Marked As Solved

sabiwara

Elixir Core Team

This actually looks like a bug (or limitation) when inferring non-atom map keys.

I opened an issue Structs nested in domain-key maps are not inferred · Issue #14915 · elixir-lang/elixir · GitHub.

Post #11

Also Liked

a-maze-d

image = %Plug.Upload{image | filename: random_filename(name)}

What about rewiting this to:

image = %{image | filename: random_filename(name)}

Post #1

garrison

Surely this is a bug? The struct is clearly typed. Maybe someone with more knowledge will reply, but I would say file an issue.

Does this infer properly?

def foo(%{"image" => image}) do
  %Plug.Upload{} = image
  %Plug.Upload{image | filename: "foo"}
end

Post #3

Awlexus

I’ve tried these 2 variations. Both trigger the warning

  defp put_random_filename(%{"image" => image} = params) do
    %Plug.Upload{filename: name} = image
    image = %Plug.Upload{image | filename: random_filename(name)}
    %{params | "image" => image}
  end

  defp put_random_filename(%{"image" => image} = params) do
    %Plug.Upload{} = image
    image = %Plug.Upload{image | filename: random_filename(image.filename)}
    %{params | "image" => image}
  end

Post #4