Keep struct fields ordered in types

Closes elixir-lang/ex_doc#1016

Author: José Valim

lib/elixir/lib/kernel/typespec.ex

@@ -557,7 +557,7 @@ defmodule Kernel.Typespec do
     types =
       :lists.map(
         fn {field, _} -> {field, Keyword.get(fields, field, quote(do: term()))} end,
-        struct
+        :lists.sort(struct)
       )
 
     fun = fn {field, _} ->

lib/elixir/test/elixir/typespec_test.exs

@@ -480,6 +480,23 @@ defmodule TypespecTest do
       assert {:type, _, :map_field_exact, [{:atom, _, :other}, {:type, _, :term, []}]} = arg2
     end
 
+    @fields Enum.map(10..42, &{:"f#{&1}", :ok})
+
+    test "@type with a large struct" do
+      bytecode =
+        test_module do
+          defstruct unquote(@fields)
+          @type my_type :: %TypespecSample{unquote_splicing(@fields)}
+        end
+
+      assert [type: {:my_type, type, []}] = types(bytecode)
+      assert {:type, _, :map, [struct, arg1, arg2 | _]} = type
+      assert {:type, _, :map_field_exact, struct_args} = struct
+      assert [{:atom, _, :__struct__}, {:atom, _, TypespecSample}] = struct_args
+      assert {:type, _, :map_field_exact, [{:atom, _, :f10}, {:atom, _, :ok}]} = arg1
+      assert {:type, _, :map_field_exact, [{:atom, _, :f11}, {:atom, _, :ok}]} = arg2
+    end
+
     test "@type with struct does not @enforce_keys" do
       bytecode =
         test_module do