Conditional child specs idea sketch

conditional_start.diff

--- a/lib/elixir/lib/supervisor.ex
+++ b/lib/elixir/lib/supervisor.ex
@@ -461,18 +461,25 @@ defmodule Supervisor do
       @behaviour Supervisor
       @opts unquote(opts)
 
+      @doc false
+      def should_start?(_arg), do: true
+
       @doc false
       def child_spec(arg) do
-        default = %{
-          id: __MODULE__,
-          start: {__MODULE__, :start_link, [arg]},
-          type: :supervisor
-        }
+        if should_start?(arg) do
+          default = %{
+            id: __MODULE__,
+            start: {__MODULE__, :start_link, [arg]},
+            type: :supervisor
+          }
 
-        Supervisor.child_spec(default, @opts)
+          Supervisor.child_spec(default, @opts)
+        else
+          nil
+        end
       end
 
-      defoverridable child_spec: 1
+      defoverridable child_spec: 1, should_start?: 0
 
       @doc false
       def init(arg)
@@ -615,7 +622,14 @@ defmodule Supervisor do
     intensity = Keyword.get(options, :max_restarts, 3)
     period = Keyword.get(options, :max_seconds, 5)
     flags = %{strategy: strategy, intensity: intensity, period: period}
-    {:ok, {flags, Enum.map(children, &init_child/1)}}
+    {:ok, {flags, Enum.reduce(children, [], &maybe_init_child/2) |> Enum.reverse()}}
+  end
+
+  defp maybe_init_child(child, acc) do
+    case init_child(child) do
+      nil -> acc
+      spec -> [spec | acc]
+    end
   end
 
   defp init_child(module) when is_atom(module) do