Add brackets around keyword lists when formatting with when, closes #13503

Author: josevalim

lib/elixir/lib/code/formatter.ex

@@ -1959,6 +1959,14 @@ defmodule Code.Formatter do
   # fn a, b, c when d -> e end
   defp clause_args_to_algebra([{:when, meta, args}], state) do
     {args, right} = split_last(args)
+
+    # If there are any keywords, wrap them in lists
+    args =
+      Enum.map(args, fn
+        [_ | _] = keyword -> {:__block__, [], [keyword]}
+        other -> other
+      end)
+
     left = {{:special, :clause_args}, meta, [args]}
     binary_op_to_algebra(:when, "when", meta, left, right, :no_parens_arg, state)
   end

lib/elixir/test/elixir/code_formatter/general_test.exs

@@ -296,6 +296,28 @@ defmodule Code.Formatter.GeneralTest do
       assert_same code, @short_length
     end
 
+    test "keeps parens if argument includes keyword list" do
+      assert_same """
+      fn [] when is_integer(x) ->
+        x + 42
+      end
+      """
+
+      bad = """
+      fn (input: x) when is_integer(x) ->
+        x + 42
+      end
+      """
+
+      good = """
+      fn [input: x] when is_integer(x) ->
+        x + 42
+      end
+      """
+
+      assert_format bad, good
+    end
+
     test "with a single clause, followed by a newline, and can fit in one line" do
       assert_same """
       fn