Quickfix for intersection protocols

This applies a quickfix solution for interesection protocols, opting to
fully ignore runtime checks of `__orig_bases__` and `__weakref__`
attributes. This has the effect of turning some false positives into
true negatives, but it also leaves some false negatives. To make that
clear, xfail test cases are added for the resulting false negatives.

A more robust fix would perhaps be to introduce recursive behavior
whenever a `__orig_bases__` attribute is found, making the type checker
check each inherited protocol individually. The resulting "top level"
intersection type would also need individual checking though, as it can
add required attributes on its own.

docs/versionhistory.rst

@@ -4,6 +4,11 @@ Version history
 This library adheres to
 `Semantic Versioning 2.0 <https://semver.org/#semantic-versioning-200>`_.
 
+**UNRELEASED**
+
+- Fixed basic support for intersection protocols
+  (`#490 <https://github.com/agronholm/typeguard/pull/490>`_; PR by @antonagestam)
+
 **4.3.0** (2024-05-27)
 
 - Added support for checking against static protocols

src/typeguard/_checkers.py

@@ -658,6 +658,8 @@ def check_protocol(
     ignored_attrs = set(dir(typing.Protocol)) | {
         "__annotations__",
         "__non_callable_proto_members__",
+        "__orig_bases__",
+        "__weakref__",
     }
     expected_methods: dict[str, tuple[Any, Any]] = {}
     expected_noncallable_members: dict[str, Any] = {}

tests/test_checkers.py

@@ -16,14 +16,17 @@
     Dict,
     ForwardRef,
     FrozenSet,
+    Iterable,
     Iterator,
     List,
     Literal,
     Mapping,
     MutableMapping,
     Optional,
+    Protocol,
     Sequence,
     Set,
+    Sized,
     TextIO,
     Tuple,
     Type,
@@ -995,6 +998,59 @@ def test_text_real_file(self, tmp_path: Path):
             check_type(f, TextIO)
 
 
+class TestIntersectingProtocol:
+    SIT = TypeVar("SIT", bound=object, covariant=True)
+
+    class SizedIterable(
+        Sized,
+        Iterable[SIT],
+        Protocol[SIT],
+    ): ...
+
+    @pytest.mark.parametrize(
+        ("subject", "predicate_type"),
+        (
+            ((), SizedIterable),
+            (range(2), SizedIterable),
+            ((), SizedIterable[int]),
+            ((1, 2, 3), SizedIterable[int]),
+            (("1", "2", "3"), SizedIterable[str]),
+        ),
+    )
+    def test_valid_member_passes(self, subject: object, predicate_type: type) -> None:
+        for _ in range(2):  # Makes sure that the cache is also exercised
+            check_type(subject, predicate_type)
+
+    xfail_nested_protocol_checks = pytest.mark.xfail(
+        reason="false negative due to missing support for nested protocol checks",
+    )
+
+    @pytest.mark.parametrize(
+        ("subject", "predicate_type"),
+        (
+            ((1 for _ in ()), SizedIterable),
+            pytest.param(
+                range(2),
+                SizedIterable[str],
+                marks=xfail_nested_protocol_checks,
+            ),
+            pytest.param(
+                (1, 2, 3),
+                SizedIterable[str],
+                marks=xfail_nested_protocol_checks,
+            ),
+            pytest.param(
+                ("1", "2", "3"),
+                SizedIterable[int],
+                marks=xfail_nested_protocol_checks,
+            ),
+        ),
+    )
+    def test_raises_for_non_member(self, subject: object, predicate_type: type) -> None:
+        with pytest.raises(TypeCheckError):
+            check_type(subject, predicate_type)
+
+
 @pytest.mark.parametrize(
     "instantiate, annotation",
     [