Do not loop when invalid block is given to formatter, closes #12922

Author: josevalim

lib/elixir/lib/code/formatter.ex

@@ -431,7 +431,7 @@ defmodule Code.Formatter do
     {color("nil", nil, state.inspect_opts), state}
   end
 
-  defp quoted_to_algebra({:__block__, meta, _} = block, _context, state) do
+  defp quoted_to_algebra({:__block__, meta, args} = block, _context, state) when is_list(args) do
     {block, state} = block_to_algebra(block, line(meta), closing_line(meta), state)
     {surround("(", block, ")"), state}
   end

lib/elixir/test/elixir/code_normalizer/quoted_ast_test.exs

@@ -226,6 +226,11 @@ defmodule Code.Normalizer.QuotedASTTest do
       assert quoted_to_string(quoted) <> "\n" == expected
     end
 
+    test "invalid block" do
+      assert quoted_to_string({:__block__, [], {:bar, [], []}}) ==
+               "{:__block__, [], {:bar, [], []}}"
+    end
+
     test "not in" do
       assert quoted_to_string(quote(do: false not in [])) == "false not in []"
     end