Introduce error category [unsafe-overload]

mypy/errorcodes.py

@@ -237,6 +237,7 @@ def __hash__(self) -> int:
     "General",
     default_enabled=False,
 )
+
 USED_BEFORE_DEF: Final[ErrorCode] = ErrorCode(
     "used-before-def", "Warn about variables that are used before they are defined", "General"
 )
@@ -261,3 +262,10 @@ def __hash__(self) -> int:
 
 # This is a catch-all for remaining uncategorized errors.
 MISC: Final = ErrorCode("misc", "Miscellaneous other checks", "General")
+
+UNSAFE_OVERLOAD: Final[ErrorCode] = ErrorCode(
+    "unsafe-overload",
+    "Warn if multiple @overload variants overlap in unsafe ways",
+    "General",
+    sub_code_of=MISC,
+)

mypy/messages.py

@@ -1601,9 +1601,10 @@ def overload_inconsistently_applies_decorator(self, decorator: str, context: Con
 
     def overloaded_signatures_overlap(self, index1: int, index2: int, context: Context) -> None:
         self.fail(
-            "Overloaded function signatures {} and {} overlap with "
-            "incompatible return types".format(index1, index2),
+            f"Overloaded function signatures {index1} and {index2} overlap with incompatible return types",
+            # "See https://mypy.readthedocs.io/en/stable/more_types.html#type-checking-the-variants.",
             context,
+            code=codes.UNSAFE_OVERLOAD,
         )
 
     def overloaded_signature_will_never_match(

mypy/types.py

@@ -3019,7 +3019,7 @@ def get_proper_type(typ: Type | None) -> ProperType | None:
 
 
 @overload
-def get_proper_types(types: list[Type] | tuple[Type, ...]) -> list[ProperType]:  # type: ignore[misc]
+def get_proper_types(types: list[Type] | tuple[Type, ...]) -> list[ProperType]:  # type: ignore[unsafe-overload]
     ...
 
 

test-data/unit/check-errorcodes.test

@@ -1072,3 +1072,17 @@ A.f = h  # type: ignore[assignment]  # E: Unused "type: ignore" comment, use nar
 [case testUnusedIgnoreEnableCode]
 # flags: --enable-error-code=unused-ignore
 x = 1  # type: ignore  # E: Unused "type: ignore" comment  [unused-ignore]
+
+[case testErrorCodeUnsafeOverloadError]
+from typing import overload, Union
+
+@overload
+def unsafe_func(x: int) -> int: ...  # E: Overloaded function signatures 1 and 2 overlap with incompatible return types  [unsafe-overload]
+@overload
+def unsafe_func(x: object) -> str: ...
+def unsafe_func(x: object) -> Union[int, str]:
+    if isinstance(x, int):
+        return 42
+    else:
+        return "some string"
+[builtins fixtures/isinstancelist.pyi]