Add tests for empty protocols on built-in objects.

One of the symptoms of the earlier problem was that you could not
check protocols against built-in types at all. This adds a test
specifically for that situation to prevent a regression.

tests/__init__.py

@@ -57,3 +57,12 @@ class RuntimeProtocol(Protocol):
     member: int
 
     def meth(self, x: str) -> None: ...
+
+
+class EmptyStaticProtocol(Protocol):
+    pass
+
+
+@runtime_checkable
+class EmptyRuntimeProtocol(Protocol):
+    pass

tests/test_checkers.py

@@ -52,6 +52,8 @@
 from . import (
     Child,
     Employee,
+    EmptyRuntimeProtocol,
+    EmptyStaticProtocol,
     JSONType,
     Parent,
     RuntimeProtocol,
@@ -1193,6 +1195,22 @@ def meth(self, x: str, y: int) -> None:
             )
 
 
+@pytest.mark.parametrize(
+    "instantiate, annotation",
+    [
+        pytest.param(True, EmptyRuntimeProtocol, id="instance_runtime"),
+        pytest.param(False, Type[EmptyRuntimeProtocol], id="class_runtime"),
+        pytest.param(True, EmptyStaticProtocol, id="instance_static"),
+        pytest.param(False, Type[EmptyStaticProtocol], id="class_static"),
+    ],
+)
+@pytest.mark.parametrize("instance_type", [object, str, Parent])
+class TestEmptyProtocol:
+    def test_empty_protocol(self, instantiate, annotation, instance_type):
+        subject = instance_type() if instantiate else instance_type
+        check_type(subject, annotation)
+
+
 class TestRecursiveType:
     def test_valid(self):
         check_type({"a": [1, 2, 3]}, JSONType)