Do not leak body inference in cond, closes #12024

Author: josevalim

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

@@ -216,6 +216,30 @@ defmodule Module.Types.Expr do
     end
   end
 
+  # cond do pat -> expr end
+  def of_expr({:cond, _meta, [[{:do, clauses}]]} = expr, _expected, stack, context) do
+    stack = push_expr_stack(expr, stack)
+
+    {result, context} =
+      reduce_ok(clauses, context, fn {:->, meta, [head, body]}, context = acc ->
+        case of_expr(head, :dynamic, stack, context) do
+          {:ok, _, context} ->
+            with {:ok, _expr_type, context} <- of_expr(body, :dynamic, stack, context) do
+              {:ok, keep_warnings(acc, context)}
+            end
+
+          error ->
+            # Skip the clause if it the head has an error
+            if meta[:generated], do: {:ok, acc}, else: error
+        end
+      end)
+
+    case result do
+      :ok -> {:ok, :dynamic, context}
+      :error -> {:error, context}
+    end
+  end
+
   # case expr do pat -> expr end
   def of_expr({:case, _meta, [case_expr, [{:do, clauses}]]} = expr, _expected, stack, context) do
     stack = push_expr_stack(expr, stack)

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

@@ -226,6 +226,31 @@ defmodule Module.Types.ExprTest do
     end
   end
 
+  describe "cond" do
+    test "do not leak body inference between clauses" do
+      assert quoted_expr(
+               [],
+               cond do
+                 1 ->
+                   b = :foo
+                   b
+
+                 2 ->
+                   b = :bar
+                   b
+               end
+             ) == {:ok, :dynamic}
+
+      assert quoted_expr(
+               [b],
+               cond do
+                 1 -> :foo = b
+                 2 -> :bar = b
+               end
+             ) == {:ok, :dynamic}
+    end
+  end
+
   test "fn" do
     assert quoted_expr(fn :foo = b -> :foo = b end) == {:ok, :dynamic}