python
@@ -1422,6 +1422,14 @@ class CustomProtocol(TestCase, Protocol):
         class CustomContextManager(typing.ContextManager, Protocol):
             pass
 
+    def test_non_runtime_protocol_isinstance_check(self):
+        class P(Protocol):
+            x: int
+
+        with self.assertRaisesRegex(TypeError, "@runtime_checkable"):
+            isinstance(1, P)
+
+
 class GenericTests(BaseTestCase):
 
     def test_basics(self):

@@ -1079,14 +1079,14 @@ def _no_init(self, *args, **kwargs):
         raise TypeError('Protocols cannot be instantiated')
 
 
-def _allow_reckless_class_cheks():
+def _allow_reckless_class_checks(depth=3):
     """Allow instance and class checks for special stdlib modules.
 
     The abc and functools modules indiscriminately call isinstance() and
     issubclass() on the whole MRO of a user class, which may contain protocols.
     """
     try:
-        return sys._getframe(3).f_globals['__name__'] in ['abc', 'functools']
+        return sys._getframe(depth).f_globals['__name__'] in ['abc', 'functools']
     except (AttributeError, ValueError):  # For platforms without _getframe().
         return True
 
@@ -1106,6 +1106,14 @@ class _ProtocolMeta(ABCMeta):
     def __instancecheck__(cls, instance):
         # We need this method for situations where attributes are
         # assigned in __init__.
+        if (
+            getattr(cls, '_is_protocol', False) and
+            not getattr(cls, '_is_runtime_protocol', False) and
+            not _allow_reckless_class_checks(depth=2)
+        ):
+            raise TypeError("Instance and class checks can only be used with"
+                            " @runtime_checkable protocols")
+
         if ((not getattr(cls, '_is_protocol', False) or
                 _is_callable_members_only(cls)) and
                 issubclass(instance.__class__, cls)):
@@ -1168,12 +1176,12 @@ def _proto_hook(other):
 
             # First, perform various sanity checks.
             if not getattr(cls, '_is_runtime_protocol', False):
-                if _allow_reckless_class_cheks():
+                if _allow_reckless_class_checks():
                     return NotImplemented
                 raise TypeError("Instance and class checks can only be used with"
                                 " @runtime_checkable protocols")
             if not _is_callable_members_only(cls):
-                if _allow_reckless_class_cheks():
+                if _allow_reckless_class_checks():
                     return NotImplemented
                 raise TypeError("Protocols with non-method members"
                                 " don't support issubclass()")

diff --git a/Misc/NEWS.d/next/Library/2021-05-12-16-43-21.bpo-38908.nM2_rO.rst b/Misc/NEWS.d/next/Library/2021-05-12-16-43-21.bpo-38908.nM2_rO.rst
@@ -0,0 +1,5 @@
+Fix issue where :mod:`typing` protocols without the  ``@runtime_checkable``
+decorator did not raise a ``TypeError`` when used with ``issubclass`` and
+``isinstance``.  Now, subclassses of ``typing.Protocol`` will raise a
+``TypeError`` when used with with those checks.
+Patch provided by Yurii Karabas.