try other names before asking user

lib/mix/tasks/phx.gen.auth.ex

@@ -294,7 +294,7 @@ defmodule Mix.Tasks.Phx.Gen.Auth do
     {_, default_scope} =
       Enum.find(existing_scopes, {nil, nil}, fn {_, scope} -> scope.default end)
 
-    key = String.to_atom(requested_scope || context.schema.singular)
+    key = String.to_atom(requested_scope || find_scope_name(context, existing_scopes))
 
     {create_new?, scope, config_string} =
       if Map.has_key?(existing_scopes, key) do
@@ -313,6 +313,36 @@ defmodule Mix.Tasks.Phx.Gen.Auth do
     }
   end
 
+  defp find_scope_name(context, existing_scopes) do
+    cond do
+      # user
+      is_new_scope?(existing_scopes, context.schema.singular) ->
+        context.schema.singular
+
+      # accounts_user
+      is_new_scope?(existing_scopes, "#{context.basename}_#{context.schema.singular}") ->
+        "#{context.basename}_#{context.schema.singular}"
+
+      # my_app_accounts_user
+      is_new_scope?(existing_scopes, "#{context.context_app}_#{context.basename}_#{context.schema.singular}") ->
+        "#{context.context_app}_#{context.basename}_#{context.schema.singular}"
+
+      true ->
+        Mix.raise """
+        Could not generate a scope name for #{context.schema.singular}! These scopes already exist:
+
+            * #{Enum.map(existing_scopes, fn {name, _scope} -> name end) |> Enum.join("\n    * ")}
+
+        You can customize the scope name by passing the --scope option.
+        """
+    end
+  end
+
+  defp is_new_scope?(existing_scopes, bin_key) do
+    key = String.to_atom(bin_key)
+    not Map.has_key?(existing_scopes, key)
+  end
+
   defp new_scope(context, key, default_scope) do
     Mix.Phoenix.Scope.new!(key, %{
       default: !default_scope,

test/mix/tasks/phx.gen.auth_test.exs

@@ -1333,7 +1333,7 @@ defmodule Mix.Tasks.Phx.Gen.AuthTest do
       end)
     end
 
-    test "when scope already exists", config do
+    test "when default scope already exists", config do
       in_tmp_phx_project(config.test, fn ->
         with_scope_env(
           :my_app,
@@ -1356,6 +1356,81 @@ defmodule Mix.Tasks.Phx.Gen.AuthTest do
               ecto_adapter: Ecto.Adapters.Postgres
             )
 
+            help_text = """
+            Your application configuration already contains a default scope: :user.
+
+            phx.gen.auth will create a new accounts_user scope.
+
+            Do you want to proceed with the generation?\
+            """
+
+            assert_received {:mix_shell, :yes?, [question]}
+            assert question == help_text
+          end
+        )
+      end)
+    end
+
+    test "when scope name cannot be generated", config do
+      in_tmp_phx_project(config.test, fn ->
+        with_scope_env(
+          :my_app,
+          [
+            user: [
+              default: true,
+              module: MyApp.Accounts.Scope,
+              assign_key: :current_scope,
+              access_path: [:user, :id],
+              schema_key: :user_id,
+              schema_type: :id,
+              schema_table: :users
+            ],
+            accounts_user: [
+              default: false,
+              module: MyApp.Accounts.Scope
+            ],
+            my_app_accounts_user: [
+              default: false,
+              module: MyApp.Accounts.Scope
+            ]
+          ],
+          fn ->
+            send(self(), {:mix_shell_input, :yes?, true})
+
+            assert_raise Mix.Error, ~r/Could not generate a scope name for user!/, fn ->
+              Gen.Auth.run(
+                ~w(Accounts User users --no-compile --live),
+                ecto_adapter: Ecto.Adapters.Postgres
+              )
+            end
+          end
+        )
+      end)
+    end
+
+    test "when given scope already exists", config do
+      in_tmp_phx_project(config.test, fn ->
+        with_scope_env(
+          :my_app,
+          [
+            user: [
+              default: true,
+              module: MyApp.Accounts.Scope,
+              assign_key: :current_scope,
+              access_path: [:user, :id],
+              schema_key: :user_id,
+              schema_type: :id,
+              schema_table: :users
+            ]
+          ],
+          fn ->
+            send(self(), {:mix_shell_input, :yes?, true})
+
+            Gen.Auth.run(
+              ~w(Accounts User users --no-compile --live --scope user),
+              ecto_adapter: Ecto.Adapters.Postgres
+            )
+
             help_text = """
             The scope user is already configured.