Added error code for overlapping function signatures (#17597)

Closes #17570. This is my first contribution to mypy! 🐍 
Added an error code for overlapping function signatures.
Test in check-errorcodes.test is a derivative of this post:
https://stackoverflow.com/q/69341607

Co-authored-by: Jelle Zijlstra <[email protected]>
Co-authored-by: Alex Waygood <[email protected]>

Author: 3 people

docs/source/error_code_list.rst

@@ -1149,6 +1149,34 @@ types you expect.
 
 See :ref:`overloading <function-overloading>` for more explanation.
 
+
+.. _code-overload-cannot-match:
+
+Check for overload signatures that cannot match [overload-cannot-match]
+--------------------------------------------------------------------------
+
+Warn if an ``@overload`` variant can never be matched, because an earlier
+overload has a wider signature. For example, this can happen if the two
+overloads accept the same parameters and each parameter on the first overload
+has the same type or a wider type than the corresponding parameter on the second
+overload.
+
+Example:
+
+.. code-block:: python
+
+    from typing import overload, Union
+
+    @overload
+    def process(response1: object, response2: object) -> object:
+        ...
+    @overload
+    def process(response1: int, response2: int) -> int: # E: Overloaded function signature 2 will never be matched: signature 1's parameter type(s) are the same or broader  [overload-cannot-match]
+        ...
+
+    def process(response1: object, response2: object) -> object:
+        return response1 + response2
+
 .. _code-annotation-unchecked:
 
 Notify about an annotation in an unchecked function [annotation-unchecked]

mypy/errorcodes.py

@@ -273,6 +273,14 @@ def __hash__(self) -> int:
 # This is a catch-all for remaining uncategorized errors.
 MISC: Final[ErrorCode] = ErrorCode("misc", "Miscellaneous other checks", "General")
 
+OVERLOAD_CANNOT_MATCH: Final[ErrorCode] = ErrorCode(
+    "overload-cannot-match",
+    "Warn if an @overload signature can never be matched",
+    "General",
+    sub_code_of=MISC,
+)
+
+
 OVERLOAD_OVERLAP: Final[ErrorCode] = ErrorCode(
     "overload-overlap",
     "Warn if multiple @overload variants overlap in unsafe ways",

mypy/messages.py

@@ -1653,6 +1653,7 @@ def overloaded_signature_will_never_match(
                 index1=index1, index2=index2
             ),
             context,
+            code=codes.OVERLOAD_CANNOT_MATCH,
         )
 
     def overloaded_signatures_typevar_specific(self, index: int, context: Context) -> None:

test-data/unit/check-errorcodes.test

@@ -1222,3 +1222,17 @@ def f(x: str) -> TypeIs[int]:  # E: Narrowed type "int" is not a subtype of inpu
     pass
 
 [builtins fixtures/tuple.pyi]
+
+
+[case testOverloadedFunctionSignature]
+from typing import overload, Union
+
+@overload
+def process(response1: float,response2: float) -> float:
+    ...
+@overload
+def process(response1: int,response2: int) -> int: # E: Overloaded function signature 2 will never be matched: signature 1's parameter type(s) are the same or broader  [overload-cannot-match]
+    ...
+
+def process(response1,response2)-> Union[float,int]:
+   return response1 + response2