Error inferring typevar on generic classes

[alfred82santa]

Bug Report

The inferred type of methods of a generic class are not bounded to typevar value on descendant classes.

To Reproduce

from typing import Iterator, Generic, TypeVar

T = TypeVar("T", str, int)

class Parent(Generic[T]):

    @classmethod
    def method_1(cls, value: T) -> T:
        return 2*value
        
    def method_2(self, value: T) -> T:
        return 2*value
        
class ChildInt(Parent[int]):
    pass

instance = ChildInt()

reveal_type(instance.method_1)  # Revealed type is "def (value: builtins.int) -> builtins.int"
reveal_type(ChildInt.method_1)  # Revealed type is "def (value: builtins.int) -> builtins.int"

reveal_type(instance.method_2)  # Revealed type is "def (value: builtins.int) -> builtins.int"
reveal_type(ChildInt.method_2)  # Revealed type is "def (self: __main__.Parent[T`1], value: T`1) -> T`1"

https://mypy-play.net/?mypy=latest&python=3.12&gist=7ce2739ccc2b43f37187d54dcdd5f923

Expected Behavior

reveal_type(ChildInt.method_2)  # Revealed type is "def (self: __main__.ChildInt, value: builtins.int) -> builtins.int"

This soulution could be acceptable, as well:

reveal_type(ChildInt.method_2)  # Revealed type is "def (self: __main__.Parent[builtins.int], value: builtins.int) -> builtins.int"

Actual Behavior

reveal_type(ChildInt.method_2)  # Revealed type is "def (self: __main__.Parent[T`1], value: T`1) -> T`1"

Your Environment

• Mypy version used: 1.15
• Mypy command-line flags: no flags
• Mypy configuration options from mypy.ini (and other config files): no config
• Python version used: 3.13

Releated issues

#15658

[A5rocks]

Does this cause any false positives or false negatives? Mypy should eventually bind self and then at that point it will find what T is. (I don't know how to interpret the related issue)

[alfred82santa]

The related problem was that T was not bound to the type in an instance object. The current problem is that T is not bound to the type in the class object.

It could produce false negative if the class method is referenced. I know it is not common and a little bit strange.

from typing import Iterator, Generic, TypeVar, Callable, Any

T = TypeVar("T", str, int)

class Parent(Generic[T]):

    @classmethod
    def method_1(cls, value: T) -> T:
        return 2*value
        
    def method_2(self, value: T) -> T:
        return 2*value
        
class ChildInt(Parent[int]):
    pass

instance = ChildInt()

reveal_type(ChildInt.method_1)  # Revealed type is "def (value: builtins.int) -> builtins.int"
reveal_type(instance.method_1)  # Revealed type is "def (value: builtins.int) -> builtins.int"

reveal_type(ChildInt.method_2)  # Revealed type is "def (self: __main__.Parent[T`1], value: T`1) -> T`1"
reveal_type(instance.method_2)  # Revealed type is "def (value: builtins.int) -> builtins.int"

def decorator[S](f: Callable[[S, int], int]) -> Callable[[S, int], int]:
    return f
    
    
class GrandChildInt(ChildInt):
    
    method_3 = decorator(ChildInt.method_2)  # error: Argument 1 to "decorator" has incompatible type "Callable[[Parent[T], T], T]"; expected "Callable[[Parent[T], int], int]"  [arg-type]

https://mypy-play.net/?mypy=latest&python=3.12&gist=7ce2739ccc2b43f37187d54dcdd5f923

[alfred82santa]

FYI: PyRigth reveal ChildInt.method_2 as def (self: __main__.ChildInt, value: builtins.int) -> builtins.int

https://pyright-play.net/?code=GYJw9gtgBALgngBwJYDsDmUkQWEMoCSMApiAIYy4A0UA4sSqUgMY0AqixAamSDQMJkANkLIAjIcRoBBFHABQ8tlAC8UDgm68AFACI2umgGcYfTChgBKRc1FGjUAAq8GMbfUYgWAbTYBdSwAuRShQqAABWzJ7CGIYAAswABN5MKgk4mAoWITkgH0ARm1bIxoAN2EAV2JA9UsoAFoAPnVgtLSQOMqQFCgAJgAqCqFq1PbQsdCMrJzEpLy%2B7SNiIWByqpq6xpa2NvGoTphu3sHh0f2oeSj7KH54pCEkggttZ06Lb1QYAL3QhGijIpUCYyChmMRVLd7o9nm5rPJOmViMI8vBNNo7g8nhYAHSzfIFepQADEUAASsQkcJiElYJxMA5dNMoNozpsxJUHjBgTivvVmlAOVyeV9dAjKcihKjONpgTBQeC8XE5oUiaSKVTJLS0RCkIzmayNrUhUJuSgjLyLPyWiazRbRYpEZLpejMTDcfj5n01eSJdTtfS9VAmZkWctVrU8nkIGRUFGcW9XL4AAYFPzrEabNip63qVNip0onWy83ysHEJW5L0%2BjWSml0zQM4MGtnGzmmkVW7aC9t2y0wMXyZkZZi4Ci4bwAZT82mAtUEInEkm8U5oX3T5m%2BuYXogkxBXk7XFg369%2BBy6PSgwEmlzSVzsDlo5BQSTd2Lcb9hQRvN89eQAzJCI5jpQIAYtC76Viq3qhKSpDgCAtTSCAaCVLEFhQAUsBgM2xCjuQoG6FA8TROYo7YBQSB7g2EK6DuS77t4iYfP47Abv4ugANxQMQAAemjMCQtJ0cIu7LkxLgsSex5Ht8RFQN4vBoA0Op%2BEAA