Infer expression types in basic operations

Author: josevalim

lib/elixir/lib/module/types.ex

@@ -285,7 +285,7 @@ defmodule Module.Types do
               Pattern.of_head(args, guards, expected, {:infer, expected}, meta, stack, context)
 
             {return_type, context} =
-              Expr.of_expr(body, stack, context)
+              Expr.of_expr(body, {Descr.term(), :ok}, stack, context)
 
             {type_index, inferred} =
               add_inferred(inferred, args_types, return_type, total - 1, [])

lib/elixir/lib/module/types/descr.ex

@@ -382,7 +382,8 @@ defmodule Module.Types.Descr do
       {dynamic, descr} =
         case :maps.take(:dynamic, descr) do
           :error -> {[], descr}
-          {dynamic, descr} -> {to_quoted(:dynamic, dynamic, opts), descr}
+          {:term, descr} -> {to_quoted(:dynamic, :term, opts), descr}
+          {dynamic, descr} -> {to_quoted(:dynamic, difference(dynamic, descr), opts), descr}
         end
 
       # Merge empty list and list together if they both exist
@@ -547,6 +548,41 @@ defmodule Module.Types.Descr do
     end
   end
 
+  @doc """
+  Returns the intersection between two types
+  only if they are compatible. Otherwise returns `:error`.
+
+  This finds the intersection between the arguments and the
+  domain of a function. It is used to refine dynamic types
+  as we traverse the program.
+  """
+  def compatible_intersection(left, right) do
+    {left_dynamic, left_static} =
+      case left do
+        :term -> {:term, :term}
+        _ -> Map.pop(left, :dynamic, left)
+      end
+
+    right_dynamic =
+      case right do
+        %{dynamic: dynamic} -> dynamic
+        _ -> right
+      end
+
+    cond do
+      empty?(left_static) ->
+        dynamic = intersection_static(unfold(left_dynamic), unfold(right_dynamic))
+        if empty?(dynamic), do: :error, else: {:ok, dynamic(dynamic)}
+
+      subtype_static?(left_static, right_dynamic) ->
+        dynamic = intersection_static(unfold(left_dynamic), unfold(right_dynamic))
+        {:ok, union(dynamic(dynamic), left_static)}
+
+      true ->
+        :error
+    end
+  end
+
   @doc """
   Optimized version of `not empty?(term(), type)`.
   """