Added module path output for the supertype in case of an error

mypy/messages.py

@@ -97,7 +97,7 @@
     get_proper_types,
 )
 from mypy.typetraverser import TypeTraverserVisitor
-from mypy.util import plural_s, unmangle
+from mypy.util import get_qualified_name, plural_s, unmangle
 
 TYPES_FOR_UNIMPORTED_HINTS: Final = {
     "typing.Any",
@@ -1379,7 +1379,7 @@ def return_type_incompatible_with_supertype(
             )
 
     def override_target(self, name: str, name_in_super: str, supertype: str) -> str:
-        target = f'supertype "{supertype}"'
+        target = f'supertype "{get_qualified_name(supertype)}"'
         if name_in_super != name:
             target = f'"{name_in_super}" of {target}'
         return target

mypy/test/test_signature_incompatible_with_supertype_error.py

@@ -0,0 +1,33 @@
+from __future__ import annotations
+
+from unittest import TestCase
+
+from mypy.util import get_qualified_name
+
+
+class TestClassA:
+    pass
+
+
+class TestClassB:
+    pass
+
+
+class TestGetQualifiedName(TestCase):
+    def test_existing_class_in_current_module(self):
+        result = get_qualified_name("TestClassA")
+        expected = f"{__name__}.TestClassA"
+        self.assertEqual(result, expected)
+
+    def test_existing_class_in_current_module_another(self):
+        result = get_qualified_name("TestClassB")
+        expected = f"{__name__}.TestClassB"
+        self.assertEqual(result, expected)
+
+    def test_non_existing_class(self):
+        result = get_qualified_name("NonExistentClass")
+        self.assertEqual(result, "NonExistentClass")
+
+    def test_empty_class_name(self):
+        result = get_qualified_name("")
+        self.assertEqual(result, "")

mypy/util.py

@@ -3,6 +3,7 @@
 from __future__ import annotations
 
 import hashlib
+import inspect
 import io
 import json
 import os
@@ -942,3 +943,14 @@ def json_loads(data: bytes) -> Any:
     if orjson is not None:
         return orjson.loads(data)
     return json.loads(data)
+
+
+def get_qualified_name(class_name: str) -> str:
+    for module_name, module in sys.modules.items():
+        if module is None:
+            continue
+        if hasattr(module, class_name):
+            cls = getattr(module, class_name)
+            if inspect.isclass(cls):
+                return f"{module.__name__}.{class_name}"
+    return class_name