Add typeshed bases to explicit type object bases

docs/changelog.md

@@ -2,6 +2,8 @@
 
 ## Unreleased
 
+- Take into account additional base classes declared in stub
+  files (fixing some false positives around `typing.IO`) (#635)
 - Fix crash on stubs that contain dict or set literals (#634)
 - Remove more old special cases and improve robustness of
   annotation parsing (#630)

pyanalyze/checker.py

@@ -163,15 +163,16 @@ def _build_type_object(self, typ: Union[type, super, str]) -> TypeObject:
         elif isinstance(typ, super):
             return TypeObject(typ, self.get_additional_bases(typ))
         else:
-            additional_bases = self.get_additional_bases(typ)
+            plugin_bases = self.get_additional_bases(typ)
+            typeshed_bases = self._get_typeshed_bases(typ)
+            additional_bases = plugin_bases | typeshed_bases
             # Is it marked as a protocol in stubs? If so, use the stub definition.
             if self.ts_finder.is_protocol(typ):
-                bases = self._get_typeshed_bases(typ)
                 return TypeObject(
                     typ,
                     additional_bases,
                     is_protocol=True,
-                    protocol_members=self._get_protocol_members(bases),
+                    protocol_members=self._get_protocol_members(typeshed_bases),
                 )
             # Is it a protocol at runtime?
             if is_instance_of_typing_name(typ, "_ProtocolMeta") and safe_getattr(

pyanalyze/test_type_object.py

@@ -374,3 +374,33 @@ def capybara(t1: Type[int], t2: type):
 
             want_hash([])  # E: incompatible_argument
             want_myhash([])  # E: incompatible_argument
+
+
+class TestIO(TestNameCheckVisitorBase):
+    @assert_passes()
+    def test_text(self):
+        from typing import TextIO
+        from typing_extensions import assert_type
+        import io
+
+        def want_io(x: TextIO):
+            x.write("hello")
+
+        def capybara():
+            with open("x") as f:
+                assert_type(f, io.TextIOWrapper)
+                want_io(f)
+
+    @assert_passes()
+    def test_binary(self):
+        from typing import BinaryIO
+        from typing_extensions import assert_type
+        import io
+
+        def want_io(x: BinaryIO):
+            x.write(b"hello")
+
+        def capybara():
+            with open("x", "rb") as f:
+                assert_type(f, io.BufferedReader)
+                want_io(f)