Properly handle list output of :unicode_util.gc/1, closes #12160

Author: josevalim

lib/elixir/lib/string.ex

@@ -1930,6 +1930,7 @@ defmodule String do
   def next_grapheme(string) when is_binary(string) do
     case :unicode_util.gc(string) do
       [gc] -> {grapheme_to_binary(gc), <<>>}
+      [gc, rest] -> {grapheme_to_binary(gc), rest}
       [gc | rest] -> {grapheme_to_binary(gc), rest}
       [] -> nil
       {:error, <<byte, rest::bits>>} -> {<<byte>>, rest}

lib/elixir/test/elixir/string_test.exs

@@ -585,6 +585,12 @@ defmodule StringTest do
   test "next_grapheme/1" do
     assert String.next_grapheme("Ā̀stute") == {"Ā̀", "stute"}
     assert String.next_grapheme("") == nil
+
+    for _ <- 1..10 do
+      bin = :crypto.strong_rand_bytes(100)
+      assert bin |> Stream.unfold(&String.next_grapheme/1) |> Enum.all?(&is_binary/1),
+             "cannot build graphemes for #{inspect(bin)}"
+    end
   end
 
   test "first/1" do