Fixed #19617 -- Refactored Form metaclasses to support more inheritance scenarios.

Thanks apollo13, funkybob and mjtamlyn for the reviews.

docs/internals/deprecation.txt

@@ -467,6 +467,8 @@ these changes.
 * The ``use_natural_keys`` argument for ``serializers.serialize()`` will be
   removed. Use ``use_natural_foreign_keys`` instead.
 
+* ``django.forms.get_declared_fields`` will be removed.
+
 2.0
 ---
 

docs/releases/1.7.txt

@@ -288,6 +288,11 @@ Forms
   :func:`~django.forms.formsets.formset_factory` to allow validating
   a minimum number of submitted forms.
 
+* The metaclasses used by ``Form`` and ``ModelForm`` have been reworked to
+  support more inheritance scenarios. The previous limitation that prevented
+  inheriting from both ``Form`` and ``ModelForm`` simultaneously have been
+  removed as long as ``ModelForm`` appears first in the MRO.
+
 Internationalization
 ^^^^^^^^^^^^^^^^^^^^
 
@@ -513,6 +518,12 @@ Miscellaneous
   still worked, even though it was not documented or officially supported. If
   you're still using it, please update to the current :setting:`CACHES` syntax.
 
+* The default ordering of ``Form`` fields in case of inheritance has changed to
+  follow normal Python MRO. Fields are now discovered by iterating through the
+  MRO in reverse with the topmost class coming last. This only affects you if
+  you relied on the default field ordering while having fields defined on both
+  the current class *and* on a parent ``Form``.
+
 Features deprecated in 1.7
 ==========================
 

docs/topics/forms/modelforms.txt

@@ -644,11 +644,12 @@ There are a couple of things to note, however.
   used. This means the child's ``Meta``, if it exists, otherwise the
   ``Meta`` of the first parent, etc.
 
-* For technical reasons, a subclass cannot inherit from both a ``ModelForm``
-  and a ``Form`` simultaneously.
+.. versionchanged:: 1.7
 
-Chances are these notes won't affect you unless you're trying to do something
-tricky with subclassing.
+* It's possible to inherit from both ``Form`` and ``ModelForm`` simultaneuosly,
+  however, you must ensure that ``ModelForm`` appears first in the MRO. This is
+  because these classes rely on different metaclasses and a class can only have
+  one metaclass.
 
 .. _modelforms-factory: