Properly handle reduce in types, closes #10224

Author: josevalim

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

@@ -277,10 +277,19 @@ defmodule Module.Types.Expr do
   # for pat <- expr do expr end
   def of_expr({:for, _meta, args} = expr, stack, context) do
     stack = push_expr_stack(expr, stack)
+    {clauses, [[{:do, block} | opts]]} = Enum.split(args, -1)
 
-    case reduce_ok(args, context, &for_clause(&1, stack, &2)) do
-      {:ok, _context} -> {:ok, :dynamic, context}
-      {:error, reason} -> {:error, reason}
+    with {:ok, context} <- reduce_ok(clauses, context, &for_clause(&1, stack, &2)),
+         {:ok, context} <- reduce_ok(opts, context, &for_option(&1, stack, &2)) do
+      if opts[:reduce] do
+        with :ok <- of_clauses(block, stack, context) do
+          {:ok, :dynamic, context}
+        end
+      else
+        with {:ok, _type, context} <- of_expr(block, stack, context) do
+          {:ok, :dynamic, context}
+        end
+      end
     end
   end
 
@@ -405,17 +414,6 @@ defmodule Module.Types.Expr do
     {:ok, context}
   end
 
-  defp for_option({:do, [{:->, _, [pattern, body]}]}, stack, context) do
-    case Pattern.of_pattern(pattern, stack, context) do
-      {:ok, _pattern_type, context} -> of_expr_context(body, stack, context)
-      {:error, reason} -> {:error, reason}
-    end
-  end
-
-  defp for_option({:do, body}, stack, context) do
-    of_expr_context(body, stack, context)
-  end
-
   defp with_clause({:<-, _, [left, expr]}, stack, context) do
     {pattern, guards} = extract_head([left])
 

lib/elixir/test/elixir/kernel/comprehension_test.exs

@@ -240,6 +240,19 @@ defmodule Kernel.ComprehensionTest do
     assert acc == %{1 => 'olleh', 3 => 'olleh'}
   end
 
+  test "for comprehensions with matched reduce" do
+    acc =
+      for entry <- [1, 2, 3], reduce: {:ok, nil} do
+        {:ok, _} ->
+          {:ok, entry}
+
+        {:error, _} = error ->
+          error
+      end
+
+    assert acc == {:ok, 3}
+  end
+
   ## List generators (inlined by the compiler)
 
   test "list for comprehensions" do