Bug Report

When overriding methods in subclasses, it's important to keep the same argument names since any argument in Python can be given as a keyword argument. If an override in a subclass changes the name of a parameter, it breaks the polymorphism for any user of the class using keyword arguments

To Reproduce

from abc import ABC, abstractmethod


class Adder(ABC):
    @abstractmethod
    def add(self, a: int, b: int) -> int:
        pass


class MyAdder(Adder):
    def add(self, a: int, b: int) -> int:
        return a + b


class OtherAdder(Adder):
    def add(self, a: int, new_b: int) -> int:
        return a + new_b


def use_adder(adder: Adder) -> int:
    return adder.add(a=1, b=2)


print(f"1 + 2 = {use_adder(MyAdder())}")  # works
print(f"1 + 2 = {use_adder(OtherAdder())}")  # broken, but type checks in mypy

Expected Behavior

The code example above should not type check, as OtherAdder is not a valid subclass of Adder. If I type check this code in pyright, it correctly identifies this issue:

$ pyright classes.py
/path/to/classes.py
  /path/to/classes.py:16:9 - error: Method "add" overrides class "Adder" in an incompatible manner
    Parameter 3 name mismatch: base parameter is named "b", override parameter is named "new_b" (reportIncompatibleMethodOverride)
1 error, 0 warnings, 0 informations

Actual Behavior

Mypy lets this error through:

$ mypy classes.py
Success: no issues found in 1 source file

Your Environment

• Mypy version used: mypy 1.15.0 (compiled: yes)
• Mypy command-line flags: None
• Mypy configuration options from mypy.ini (and other config files): None
• Python version used: Python 3.13.3