Attempt to reproduce #91

mypy-zope.ini

@@ -1,3 +1,4 @@
 [mypy]
 namespace_packages=True
+warn_unreachable=True
 plugins=mypy_zope:plugin

src/mypy_zope/plugin.py

@@ -3,6 +3,7 @@
 from typing import Type as PyType
 from typing import cast
 
+from mypy.typestate import type_state
 from mypy.types import (
     Type,
     Instance,
@@ -722,6 +723,13 @@ def _apply_interface(self, impl: TypeInfo, iface: TypeInfo) -> None:
         if promote not in impl._promote:
             impl._promote.append(promote)
 
+        # Remember implementation as a subtype of an interface. HACK: we are
+        # writing to a global variable here, so potentially this might be a
+        # memory leak. Needs testing with a large codebase.
+        asmpt = (Instance(impl, []), promote)
+        type_state.get_assumptions(False).append(asmpt)
+        type_state.get_assumptions(True).append(asmpt)
+
 
 def plugin(version: str) -> PyType[Plugin]:
     return ZopeInterfacePlugin

tests/samples/dmr_unreachable.py

@@ -0,0 +1,31 @@
+from typing import Optional
+
+from zope.interface import Interface, implementer
+
+
+class IFoo(Interface):
+    pass
+
+
+@implementer(IFoo)
+class BaseFoo:
+    pass
+
+
+class ChildFoo(BaseFoo):
+    pass
+
+
+class IFooFactory(Interface):
+    def build() -> Optional[IFoo]:
+        pass
+
+
+def build_and_use_foo(client_factory: IFooFactory) -> None:
+    client_protocol = client_factory.build()
+    assert isinstance(client_protocol, ChildFoo)
+    print("Hello")
+
+
+"""
+"""

tests/samples/isinstance_impl.py

@@ -0,0 +1,29 @@
+from typing import Optional
+from zope.interface import implementer, Interface
+
+
+class IFoo(Interface):
+    ...
+
+
+@implementer(IFoo)
+class MyFoo:
+    ...
+
+
+def make_foo() -> Optional[IFoo]:
+    return MyFoo()
+
+
+x = make_foo()
+reveal_type(x)
+assert isinstance(x, MyFoo)
+
+# The code below should not be considered unreachable
+print("hello")
+
+"""
+<output>
+isinstance_impl.py:19: note: Revealed type is "Union[__main__.IFoo, None]"
+</output>
+"""

tests/test_samples.py

@@ -26,6 +26,7 @@ def test_samples(samplefile, mypy_cache_dir):
     opts.show_traceback = True
     opts.namespace_packages = True
     opts.hide_error_codes = True
+    opts.warn_unreachable = True
     opts.plugins = ['mypy_zope:plugin']
     # Config file is needed to load plugins, it doesn't not exist and is not
     # supposed to.