@@ -3448,8 +3448,8 @@ def meth(self): pass
 
         self.assertNotIn("__protocol_attrs__", vars(NonP))
         self.assertNotIn("__protocol_attrs__", vars(NonPR))
-        self.assertNotIn("__callable_proto_members_only__", vars(NonP))
-        self.assertNotIn("__callable_proto_members_only__", vars(NonPR))
+        self.assertNotIn("__non_callable_proto_members__", vars(NonP))
+        self.assertNotIn("__non_callable_proto_members__", vars(NonPR))
 
         self.assertEqual(get_protocol_members(P), {"x"})
         self.assertEqual(get_protocol_members(PR), {"meth"})
@@ -3472,6 +3472,12 @@ def meth(self): pass
             vars(NonPR).keys(), vars(D).keys() | acceptable_extra_attrs
         )
 
+    def test_nonruntime_protocols_not_unnecessarily_introspected(self) -> None:
+        class Foo(Protocol):
+            x = 1
+
+        self.assertFalse(hasattr(Foo, "__non_callable_proto_members__"))
+
     def test_custom_subclasshook(self):
         class P(Protocol):
             x = 1
@@ -4105,6 +4111,7 @@ def method(self) -> None: ...
         self.assertNotIsInstance(42, ProtocolWithMixedMembers)
 
     def test_protocol_issubclass_error_message(self):
+        @runtime_checkable
         class Vec2D(Protocol):
             x: float
             y: float
@@ -4120,6 +4127,37 @@ def square_norm(self) -> float:
         with self.assertRaisesRegex(TypeError, re.escape(expected_error_message)):
             issubclass(int, Vec2D)
 
+    def test_nonruntime_protocol_interaction_with_evil_classproperty(self):
+        class classproperty:
+            def __get__(self, instance, type):
+                raise RuntimeError("NO")
+
+        class Commentable(Protocol):
+            evil = classproperty()
+
+        # recognised as a protocol attr,
+        # but not actually accessed for non-runtime protocols
+        self.assertEqual(get_protocol_members(Commentable), {"evil"})
+
+    def test_runtime_protocol_interaction_with_evil_classproperty(self):
+        class CustomError(Exception): pass
+
+        class classproperty:
+            def __get__(self, instance, type):
+                raise CustomError
+
+        with self.assertRaises(TypeError) as cm:
+            @runtime_checkable
+            class Commentable(Protocol):
+                evil = classproperty()
+
+        exc = cm.exception
+        self.assertEqual(
+            exc.args[0],
+            "Failed to determine whether protocol member 'evil' is a method member"
+        )
+        self.assertIs(type(exc.__cause__), CustomError)
+
 
 class GenericTests(BaseTestCase):
 
 
@@ -1670,7 +1670,7 @@ class _TypingEllipsis:
 _TYPING_INTERNALS = frozenset({
     '__parameters__', '__orig_bases__',  '__orig_class__',
     '_is_protocol', '_is_runtime_protocol', '__protocol_attrs__',
-    '__callable_proto_members_only__', '__type_params__',
+    '__non_callable_proto_members__', '__type_params__',
 })
 
 _SPECIAL_NAMES = frozenset({
@@ -1833,11 +1833,6 @@ def __init__(cls, *args, **kwargs):
         super().__init__(*args, **kwargs)
         if getattr(cls, "_is_protocol", False):
             cls.__protocol_attrs__ = _get_protocol_attrs(cls)
-            # PEP 544 prohibits using issubclass()
-            # with protocols that have non-method members.
-            cls.__callable_proto_members_only__ = all(
-                callable(getattr(cls, attr, None)) for attr in cls.__protocol_attrs__
-            )
 
     def __subclasscheck__(cls, other):
         if cls is Protocol:
@@ -1846,25 +1841,22 @@ def __subclasscheck__(cls, other):
             getattr(cls, '_is_protocol', False)
             and not _allow_reckless_class_checks()
         ):
+            if not getattr(cls, '_is_runtime_protocol', False):
+                _type_check_issubclass_arg_1(other)
+                raise TypeError(
+                    "Instance and class checks can only be used with "
+                    "@runtime_checkable protocols"
+                )
             if (
-                not cls.__callable_proto_members_only__
+                cls.__non_callable_proto_members__
                 and cls.__dict__.get("__subclasshook__") is _proto_hook
             ):
                 _type_check_issubclass_arg_1(other)
-                non_method_attrs = sorted(
-                    attr for attr in cls.__protocol_attrs__
-                    if not callable(getattr(cls, attr, None))
-                )
+                non_method_attrs = sorted(cls.__non_callable_proto_members__)
                 raise TypeError(
                     "Protocols with non-method members don't support issubclass()."
                     f" Non-method members: {str(non_method_attrs)[1:-1]}."
                 )
-            if not getattr(cls, '_is_runtime_protocol', False):
-                _type_check_issubclass_arg_1(other)
-                raise TypeError(
-                    "Instance and class checks can only be used with "
-                    "@runtime_checkable protocols"
-                )
         return _abc_subclasscheck(cls, other)
 
     def __instancecheck__(cls, instance):
@@ -1892,7 +1884,7 @@ def __instancecheck__(cls, instance):
                 val = getattr_static(instance, attr)
             except AttributeError:
                 break
-            if val is None and callable(getattr(cls, attr, None)):
+            if val is None and attr not in cls.__non_callable_proto_members__:
                 break
         else:
             return True
@@ -2114,6 +2106,22 @@ def close(self): ...
         raise TypeError('@runtime_checkable can be only applied to protocol classes,'
                         ' got %r' % cls)
     cls._is_runtime_protocol = True
+    # PEP 544 prohibits using issubclass()
+    # with protocols that have non-method members.
+    # See gh-113320 for why we compute this attribute here,
+    # rather than in `_ProtocolMeta.__init__`
+    cls.__non_callable_proto_members__ = set()
+    for attr in cls.__protocol_attrs__:
+        try:
+            is_callable = callable(getattr(cls, attr, None))
+        except Exception as e:
+            raise TypeError(
+                f"Failed to determine whether protocol member {attr!r} "
+                "is a method member"
+            ) from e
+        else:
+            if not is_callable:
+                cls.__non_callable_proto_members__.add(attr)
     return cls
 
 
 
@@ -0,0 +1,4 @@
+Fix regression in Python 3.12 where :class:`~typing.Protocol` classes that
+were not marked as :func:`runtime-checkable <typing.runtime_checkable>`
+would be unnecessarily introspected, potentially causing exceptions to be
+raised if the protocol had problematic members. Patch by Alex Waygood.