Fix implementation checks on generic interfaces

CHANGELOG.rst

@@ -2,6 +2,28 @@
 ------------------
 
 * Added support for Python 3.10.
+* #109: Non-generic implementers of generic interfaces are now correctly checked.
+
+  Previously checking if a non-generic class implements a generic interface always used to fail, as
+  the latter has a special method (``__class_getitem__``) that the former does't have, which led to
+  false negatives.
+
+  .. code-block:: python
+
+      class IFoo(Interface, typing.Generic[T]):
+          def Bar(self) -> T:
+              ...
+
+
+      @ImplementsInterface(IFoo, no_check=True)
+      class Foo:
+          @Implements(IFoo.Bar)
+          def Bar(self) -> str:
+              return "baz"
+
+
+      AssertImplements(Foo, IFoo)  # Ok now.
+
 
 2.1.0 (2021-03-19)
 ------------------

src/oop_ext/interface/_interface.py

@@ -747,7 +747,7 @@ def AssertImplementsFullChecking(
 
 
 # set of methods that might be declared in interfaces but should be not be required by implementations
-_INTERFACE_METHODS_TO_IGNORE = {"__init_subclass__"}
+_INTERFACE_METHODS_TO_IGNORE = {"__init_subclass__", "__class_getitem__"}
 
 
 def _AssertImplementsFullChecking(

src/oop_ext/interface/_tests/test_interface.py

@@ -1077,3 +1077,39 @@ def AfterCaption(*args):
 
     assert Foo.GetCaption() == "Foo"
     assert Foo().GetValues("m") == [0.1, 10.0]
+
+
+def testGenericInterface() -> None:
+    """Generic interfaces need to support checking on both generic and non-generic implementers"""
+    from typing import FrozenSet, Generic, TypeVar
+
+    T = TypeVar("T", covariant=True)
+
+    class IFoo(Interface, Generic[T], TypeCheckingSupport):
+        # Class instance defined here as a workaround for this class to work in Python 3.6.
+        __abstractmethods__: FrozenSet = frozenset()
+
+        def GetOutput(self) -> T:  # type:ignore[empty-body]
+            ...
+
+    @ImplementsInterface(IFoo, no_check=True)  # Will check later.
+    class GenericFoo(Generic[T]):
+        def __init__(self, output: T) -> None:
+            self.output = output
+
+        @Implements(IFoo.GetOutput)
+        def GetOutput(self) -> T:
+            return self.output
+
+    @ImplementsInterface(IFoo, no_check=True)
+    class NonGenericFoo:
+        @Implements(IFoo.GetOutput)
+        def GetOutput(self) -> int:
+            return 1
+
+    # This works out of the box.
+    AssertImplements(GenericFoo, IFoo)
+    AssertImplements(GenericFoo[str](output="foo"), IFoo)
+
+    # This only works if we skip the verification of `__class_getitem__` method.
+    AssertImplements(NonGenericFoo, IFoo)