Handle more unicode sequences in split_at, closes #11617

Author: josevalim

lib/elixir/lib/string.ex

@@ -2723,12 +2723,16 @@ defmodule String do
             graphemes_and_length: 1,
             reverse_characters_to_binary: 1}
 
-  defp byte_size_remaining_at(binary, 0) do
-    byte_size(binary)
+  defp byte_size_unicode(binary) when is_binary(binary), do: byte_size(binary)
+  defp byte_size_unicode([head]), do: byte_size_unicode(head)
+  defp byte_size_unicode([head | tail]), do: byte_size_unicode(head) + byte_size_unicode(tail)
+
+  defp byte_size_remaining_at(unicode, 0) do
+    byte_size_unicode(unicode)
   end
 
-  defp byte_size_remaining_at(binary, n) do
-    case :unicode_util.gc(binary) do
+  defp byte_size_remaining_at(unicode, n) do
+    case :unicode_util.gc(unicode) do
       [_] -> 0
       [_ | rest] -> byte_size_remaining_at(rest, n - 1)
       [] -> 0

lib/elixir/test/elixir/string_test.exs

@@ -131,6 +131,11 @@ defmodule StringTest do
     end
   end
 
+  test "split_at/2 with invalid guard" do
+    assert String.split_at(<<?a, 195, 10, ?a>>, 2) == {<<?a, 195>>, <<10, ?a>>}
+    assert String.split_at(<<107, 205, 135, 184>>, 1) == {<<107, 205, 135>>, <<184>>}
+  end
+
   test "upcase/1" do
     assert String.upcase("123 abcd 456 efg hij ( %$#) kl mnop @ qrst = -_ uvwxyz") ==
              "123 ABCD 456 EFG HIJ ( %$#) KL MNOP @ QRST = -_ UVWXYZ"