Do not reorder starting classes

Author: José Valim

lib/elixir/test/elixir/string_test.exs

@@ -321,6 +321,8 @@ defmodule StringTest do
     assert String.normalize("거̄", :nfc) == "거̄"
     assert String.normalize("뢴", :nfc) == "뢴"
 
+    ## Cases from NormalizationTest.txt
+
     # 05B8 05B9 05B1 0591 05C3 05B0 05AC 059F
     # 05B1 05B8 05B9 0591 05C3 05B0 05AC 059F
     # HEBREW POINT QAMATS, HEBREW POINT HOLAM, HEBREW POINT HATAF SEGOL,
@@ -343,6 +345,11 @@ defmodule StringTest do
     # 0308 0301
     # COMBINING GREEK DIALYTIKA TONOS
     assert String.normalize("\u0344", :nfc) == "\u0308\u0301"
+
+    # 115B9 0334 115AF
+    # 115B9 0334 115AF
+    # SIDDHAM VOWEL SIGN AI, COMBINING TILDE OVERLAY, SIDDHAM VOWEL SIGN AA
+    assert String.normalize("𑖹̴𑖯", :nfc) == "𑖹̴𑖯"
   end
 
   test "graphemes" do

lib/elixir/unicode/unicode.ex

@@ -451,6 +451,7 @@ defmodule String.Break do
   defp add_buffer_to_acc(buffer, acc), do: [buffer|acc]
 end
 
+
 defmodule String.Normalizer do
   @moduledoc false
 
@@ -521,12 +522,19 @@ defmodule String.Normalizer do
     end
   end
   defp canonical_order(<<h::utf8, t::binary>>, acc) do
-    canonical_order(t, [{h, combining_class(h)}|acc])
+    case combining_class(h) do
+      0 -> canonical_order(acc) <> canonical_order(t, [{h, 0}])
+      n -> canonical_order(t, [{h, n}|acc])
+    end
+  end
+  defp canonical_order(<<>>, acc) do
+    canonical_order(acc)
   end
-  defp canonical_order(<<>>, [{x, _}]) do
+
+  defp canonical_order([{x, _}]) do
     <<x::utf8>>
   end
-  defp canonical_order(<<>>, acc) do
+  defp canonical_order(acc) do
     :lists.keysort(2, Enum.reverse(acc))
     |> Enum.map(&<<elem(&1, 0)::utf8>>)
     |> IO.iodata_to_binary