Nested form with hidden inputs raises ArgumentError

.gitpod.yml

@@ -0,0 +1,5 @@
+image: elixir:latest
+
+tasks:
+  - init: mix deps.get && mix compile
+    command: iex -S mix

test/phoenix_test/live_test.exs

@@ -421,6 +421,20 @@ defmodule PhoenixTest.LiveTest do
       |> assert_has("#full-form option[value='elf']")
     end
 
+    test "selects option in a nested form generated by inputs_for", %{conn: conn} do
+      # ** (ArgumentError) value for hidden "elf[cloak][_touched]" must be one of ["_form_type,_persistent_id,_touched,name,type"], got: "color,type"
+      # code: |> select("Small", from: "Elf Cloak Size")
+      # stacktrace:
+      #   (phoenix_live_view 0.20.17) lib/phoenix_live_view/test/live_view_test.ex:1102: Phoenix.LiveViewTest.call/2
+      #   (phoenix_test 0.3.1) lib/phoenix_test/live.ex:219: PhoenixTest.Live.fill_form/3
+
+      conn
+      |> visit("/live/index")
+      |> fill_in("Elf Cloak Color", with: "Green")
+      |> select("Small", from: "Elf Cloak Size")
+      |> assert_has("#nested-form-with-hidden-inputs option[value='small']")
+    end
+
     test "works in 'nested' forms", %{conn: conn} do
       conn
       |> visit("/live/index")

test/support/index_live.ex

@@ -101,6 +101,19 @@ defmodule PhoenixTest.IndexLive do
       Save Owner Form
     </button>
 
+    <form id="nested-form-with-hidden-inputs" phx-submit="save-form" phx-change="validate">
+      <input type="hidden" name="elf[cloak][_persistent_id]" value="0">
+      <input type="hidden" name="elf[cloak][_touched]" value="">
+      <input type="hidden" name="elf[cloak][_form_type]" value="create">
+      <label for="elf_cloak_color">Elf Cloak Color</label>
+      <input type="text" name="elf[cloak][color]" id="elf_cloak_color" phx-debounce="300">
+      <label for="elf_cloak_size">Elf Cloak Size</label>
+      <select name="elf[cloak][size]" id="elf_cloak_size">
+        <option value="small">Small</option>
+        <option value="large">Large</option>
+      </select>
+    </form>
+
     <form id="nested-form" phx-submit="save-form">
       <label for="user-name">User Name</label>
       <input id="user-name" name="user[name]" />
@@ -343,6 +356,10 @@ defmodule PhoenixTest.IndexLive do
     }
   end
 
+  def handle_event("validate", _form_data, socket) do
+    {:noreply, socket}
+  end
+
   def handle_event("save-redirect-form", _, socket) do
     {:noreply, push_navigate(socket, to: "/live/page_2")}
   end