fix: handle builtin types in unions (#752)

.gitpod.yml

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

lib/ash/type/union.ex

@@ -521,6 +521,7 @@ defmodule Ash.Type.Union do
          {:ok, type_config} <- Keyword.fetch(type_configs, type_name),
          {:ok, type} <- Keyword.fetch(type_config, :type),
          type_constraints <- Keyword.get(type_config, :constraints, []),
+         type <- Ash.Type.get_type(type),
          {:ok, new_value} <- type.handle_change(nil, new_value, type_constraints) do
       {:ok, %Ash.Union{type: type_name, value: new_value}}
     end
@@ -574,6 +575,7 @@ defmodule Ash.Type.Union do
          {:ok, type_config} <- Keyword.fetch(type_configs, type_name),
          {:ok, type} <- Keyword.fetch(type_config, :type),
          type_constraints <- Keyword.get(type_config, :constraints, []),
+         type <- Ash.Type.get_type(type),
          {:ok, value} <- type.prepare_change(old_value, new_value, type_constraints) do
       {:ok, %Ash.Union{type: type_name, value: value}}
     end

test/type/union_test.exs

@@ -84,6 +84,62 @@ defmodule Ash.Test.Type.UnionTest do
     assert {:error, _} = Ash.Type.cast_input(:union, 11, constraints)
   end
 
+  test "it handles changes between native types" do
+    constraints = [
+      types: [
+        foo: [
+          type: :integer
+        ],
+        bar: [
+          type: :string
+        ]
+      ]
+    ]
+
+    assert {:ok, %Ash.Union{type: :bar, value: "bar"}} =
+             Ash.Type.Union.prepare_change(
+               %Ash.Union{type: :foo, value: 1},
+               "bar",
+               constraints
+             )
+
+    assert {:ok, %Ash.Union{type: :bar, value: "bar"}} =
+             Ash.Type.Union.handle_change(
+               %Ash.Union{type: :foo, value: 1},
+               %Ash.Union{type: :bar, value: "bar"},
+               constraints
+             )
+  end
+
+  test "it handles changes between native and tagged types" do
+    constraints = [
+      types: [
+        foo: [
+          type: Foo,
+          tag: :type,
+          tag_value: :foo
+        ],
+        bar: [
+          type: :string
+        ]
+      ]
+    ]
+
+    assert {:ok, %Ash.Union{type: :bar, value: "bar"}} =
+             Ash.Type.Union.prepare_change(
+               %Ash.Union{type: :foo, value: %Foo{}},
+               "bar",
+               constraints
+             )
+
+    assert {:ok, %Ash.Union{type: :bar, value: "bar"}} =
+             Ash.Type.Union.handle_change(
+               %Ash.Union{type: :foo, value: %{}},
+               %Ash.Union{type: :bar, value: "bar"},
+               constraints
+             )
+  end
+
   test "it handles tagged types" do
     constraints = [
       types: [