Fixed TypeError when trying to check against a naked str forward refe…

…rence

docs/versionhistory.rst

@@ -8,6 +8,9 @@ This library adheres to `Semantic Versioning 2.0 <https://semver.org/#semantic-v
 - Added ``InstrumentationWarning`` to the public API
 - Fixed instrumentation using ``typing.Optional`` without a subscript when the subscript
   value was erased due to being an ignored import
+- Fixed ``TypeError: isinstance() arg 2 must be a type or tuple of types`` when
+  instrumented code tries to check a value against a naked (``str``, not ``ForwardRef``)
+  forward reference
 
 **4.0.0rc4** (2023-04-15)
 

src/typeguard/_checkers.py

@@ -756,8 +756,16 @@ def check_type_internal(
             checker(value, origin_type, args, memo)
             return
 
-    if not isinstance(value, origin_type):
-        raise TypeCheckError(f"is not an instance of {qualified_name(origin_type)}")
+    if isclass(origin_type):
+        if not isinstance(value, origin_type):
+            raise TypeCheckError(f"is not an instance of {qualified_name(origin_type)}")
+    elif type(origin_type) is str:
+        warnings.warn(
+            f"Skipping type check against {origin_type!r}; this looks like a "
+            f"string-form forward reference imported from another module",
+            TypeHintWarning,
+            stacklevel=get_stacklevel(),
+        )
 
 
 # Equality checks are applied to these

tests/test_checkers.py

@@ -35,8 +35,10 @@
     CollectionCheckStrategy,
     ForwardRefPolicy,
     TypeCheckError,
+    TypeCheckMemo,
     TypeHintWarning,
     check_type,
+    check_type_internal,
     suppress_type_checks,
 )
 from typeguard._utils import qualified_name
@@ -1037,3 +1039,11 @@ def test_none():
 def test_return_checked_value():
     value = {"foo": 1}
     assert check_type(value, Dict[str, int]) is value
+
+
+def test_imported_str_forward_ref():
+    value = {"foo": 1}
+    memo = TypeCheckMemo(globals(), locals())
+    pattern = r"Skipping type check against 'Dict\[str, int\]'"
+    with pytest.warns(TypeHintWarning, match=pattern):
+        check_type_internal(value, "Dict[str, int]", memo)