Use TypeGuard in IsImplementation

However due to python/mypy#4717, this breaks
IsImplemenation with errors like this one:

   Only concrete class can be given where "Type[_InterfM3]" is expected  [type-abstract]

Which is unfortunate.

CHANGELOG.rst

@@ -2,6 +2,10 @@
 ------------------
 
 * Added support for Python 3.10.
+* ``interface.IsImplementation`` now uses `TypeGuard`_ so ``mypy`` understands that if it returns ``True``, the
+  passed object can be considered an implementation of that interface.
+
+.. _TypeGuard: https://docs.python.org/3/library/typing.html#typing.TypeGuard
 
 2.1.0 (2021-03-19)
 ------------------

src/oop_ext/interface/_interface.py

@@ -42,7 +42,6 @@ class MyCalculatorImpl(object):
 from functools import lru_cache
 from typing import Any
 from typing import Callable
-from typing import cast
 from typing import Dict
 from typing import FrozenSet
 from typing import Generic
@@ -57,6 +56,8 @@ class MyCalculatorImpl(object):
 from typing import TypeVar
 from typing import Union
 
+from typing_extensions import TypeGuard
+
 from oop_ext.foundation.cached_method import ImmutableParamsCachedMethod
 from oop_ext.foundation.decorators import Deprecated
 from oop_ext.foundation.is_frozen import IsDevelopment
@@ -99,7 +100,7 @@ class BadImplementationError(InterfaceError):
     pass
 
 
-# InterfaceType should be changed to ``Type[Interface]`` after https://github.com/python/mypy/issues/5374
+# InterfaceType should be changed to ``Type[Interface]`` after https://github.com/python/mypy/issues/4717
 # is fixed.
 InterfaceType = Type[Any]
 
@@ -314,10 +315,10 @@ def _CheckIsInterfaceSubclass(interface: Any) -> None:
 
 def IsImplementation(
     class_or_instance: Any,
-    interface: InterfaceType,
+    interface: Type[T],
     *,
     requires_declaration: bool = True,
-) -> bool:
+) -> TypeGuard[T]:
     """
     :param class_or_instance: type or classobj or object
 

src/oop_ext/interface/_tests/test_interface.py

@@ -1019,6 +1019,35 @@ def Foo(a: IAcme) -> int:
     result.assert_ok()
 
 
+def testIsImplementation(type_checker) -> None:
+    """
+    Check that IsImplementation correctly tells mypy that the type now has that narrowed type
+    inside the if-block.
+    """
+    type_checker.make_file(
+        """
+        from oop_ext.interface import Interface, IsImplementation
+        class IAcme(Interface):
+            def Foo(self, a, b=None) -> int:  # type:ignore[empty-body]
+                ...
+
+        class Acme:
+            def Foo(self, a, b=None) -> int:
+                return 40 + a
+
+        def GetIt(a: object) -> int:
+            if IsImplementation(a, IAcme):
+                return a.Foo(10)
+            return 0
+
+        GetIt(Acme())
+        GetIt("hello")
+        """
+    )
+    result = type_checker.run()
+    result.assert_ok()
+
+
 def testAttributeTypeChecking(type_checker) -> None:
     class IFoo(Interface, TypeCheckingSupport):
         value: int = Attribute(int)