Ensure proper @type is generated for dynamic struct

Author: José Valim

lib/elixir/lib/kernel.ex

@@ -3316,6 +3316,15 @@ defmodule Kernel do
 
     types =
       case bootstraped?(Kernel.Typespec) do
+        true when types == [] ->
+          quote unquote: false do
+            unless Kernel.Typespec.defines_type?(__MODULE__, :t, 0) do
+              types = Enum.map(fields, fn {key, _} ->
+                {key, quote(do: term)}
+              end)
+              @type t :: %{unquote_splicing(types), __struct__: __MODULE__}
+            end
+          end
         true ->
           quote do
             unless Kernel.Typespec.defines_type?(__MODULE__, :t, 0) do
@@ -3326,7 +3335,10 @@ defmodule Kernel do
           nil
       end
 
-    [types, fields]
+    quote do
+      unquote(fields)
+      unquote(types)
+    end
   end
 
   @doc ~S"""

lib/elixir/test/elixir/kernel/typespec_test.exs

@@ -282,6 +282,20 @@ defmodule Kernel.TypespecTest do
            ]}, []}] = types
   end
 
+  test "@type from dynamic structs" do
+    types = test_module do
+      fields = [name: nil, age: 0]
+      defstruct fields
+      @type
+    end
+
+    assert [{:t, {:type, _, :map, [
+              {:type, _, :map_field_assoc, {:atom, _, :name}, {:type, _, :term, []}},
+              {:type, _, :map_field_assoc, {:atom, _, :age}, {:type, _, :term, []}},
+              {:type, _, :map_field_assoc, {:atom, _, :__struct__}, {:atom, _, Kernel.TypespecTest.T}}
+           ]}, []}] = types
+  end
+
   test "@type unquote fragment" do
     spec = test_module do
       quoted = quote unquote: false do