Special case the base Interface in @implementer/classImplements to skip the redundancy detection for the sake of plone.app.caching.

docs/README.rst

@@ -518,7 +518,7 @@ declarations. Here's a silly example:
   ...     zope.interface.implementedBy(Foo),
   ...     ISpecial,
   ... )
-  ... class Special2(Foo):
+  ... class Special2(object):
   ...     reason = 'I just am'
   ...     def brag(self):
   ...         return "I'm special because %s" % self.reason

src/zope/interface/declarations.py

@@ -33,6 +33,7 @@ from types import ModuleType
 import weakref
 
 from zope.interface.advice import addClassAdvisor
+from zope.interface.interface import Interface
 from zope.interface.interface import InterfaceClass
 from zope.interface.interface import SpecificationBase
 from zope.interface.interface import Specification
@@ -470,9 +471,6 @@ def classImplements(cls, *interfaces):
     # order, while still allowing for BWC (in the past, we always
     # appended)
     for iface in interfaces:
-        if spec.isOrExtends(iface):
-            continue
-
         for b in spec.declared:
             if iface.extends(b):
                 before.append(iface)
@@ -493,18 +491,34 @@ def classImplementsFirst(cls, iface):
     .. versionadded:: 5.0.0
     """
     spec = implementedBy(cls)
-    if not spec.isOrExtends(iface):
-        _classImplements_ordered(spec, (iface,), ())
+    _classImplements_ordered(spec, (iface,), ())
 
 
 def _classImplements_ordered(spec, before=(), after=()):
+    # Elide everything already inherited.
+    # Except, if it is the root, and we don't already declare anything else
+    # that would imply it, allow the root through. (TODO: When we disallow non-strict
+    # IRO, this part of the check can be removed because it's not possible to re-declare
+    # like that.)
+    before = [
+        x
+        for x in before
+        if not spec.isOrExtends(x) or (x is Interface and not spec.declared)
+    ]
+    after = [
+        x
+        for x in after
+        if not spec.isOrExtends(x) or (x is Interface and not spec.declared)
+    ]
+
     # eliminate duplicates
     new_declared = []
     seen = set()
-    for b in before + spec.declared + after:
-        if b not in seen:
-            new_declared.append(b)
-            seen.add(b)
+    for l in before, spec.declared, after:
+        for b in l:
+            if b not in seen:
+                new_declared.append(b)
+                seen.add(b)
 
     spec.declared = tuple(new_declared)
 

src/zope/interface/tests/test_declarations.py

@@ -890,6 +890,25 @@ class Test_classImplements(unittest.TestCase):
             pass
         self.__check_implementer_redundant(Foo)
 
+    def test_redundant_implementer_Interface(self):
+        from zope.interface import Interface
+        from zope.interface import implementedBy
+        from zope.interface import ro
+        from zope.interface.tests.test_ro import C3Setting
+
+        class Foo(object):
+            pass
+
+        with C3Setting(ro.C3.STRICT_IRO, False):
+            self._callFUT(Foo, Interface)
+            self.assertEqual(list(implementedBy(Foo)), [Interface])
+
+            class Baz(Foo):
+                pass
+
+            self._callFUT(Baz, Interface)
+            self.assertEqual(list(implementedBy(Baz)), [Interface])
+
     def _order_for_two(self, applied_first, applied_second):
         return (applied_first, applied_second)
 

src/zope/interface/tests/test_registry.py

@@ -873,7 +873,7 @@ class ComponentsTests(unittest.TestCase):
         ibar = IFoo('IBar')
         _info = u'info'
         _name = u'name'
-        _to_reg = object()
+
         class _Factory(object):
             pass