Infer return types for for+:reduce

Author: josevalim

lib/elixir/lib/module/types/expr.ex

@@ -346,9 +346,10 @@ defmodule Module.Types.Expr do
     # We handle reduce and into accordingly instead.
     if Keyword.has_key?(opts, :reduce) do
       reduce = Keyword.fetch!(opts, :reduce)
-      {_, context} = of_expr(reduce, stack, context)
-      {_, context} = of_clauses(block, [dynamic()], :for_reduce, stack, {none(), context})
-      {dynamic(), context}
+      {reduce_type, context} = of_expr(reduce, stack, context)
+      # TODO: We need to type check against dynamic() instead of using reduce_type
+      # because this is recursive. We need to infer the block type first.
+      of_clauses(block, [dynamic()], :for_reduce, stack, {reduce_type, context})
     else
       into = Keyword.get(opts, :into, [])
       {into_wrapper, gradual?, context} = for_into(into, meta, stack, context)

lib/elixir/lib/module/types/pattern.ex

@@ -838,7 +838,7 @@ defmodule Module.Types.Pattern do
     end
   end
 
-  defp badpattern(:default, pattern_or_expr, _) do
+  defp badpattern(_, pattern_or_expr, _) do
     {pattern_or_expr,
      """
      the following pattern will never match:

lib/elixir/test/elixir/module/types/expr_test.exs

@@ -1460,6 +1460,16 @@ defmodule Module.Types.ExprTest do
                )
              ) == dynamic(union(binary(), list(float())))
     end
+
+    test ":reduce" do
+      assert typecheck!(
+               [list],
+               for _ <- list, reduce: :ok do
+                 :ok -> 1
+                 _ -> 2.0
+               end
+             ) == union(atom([:ok]), union(integer(), float()))
+    end
   end
 
   describe "info" do