(🎁) Support type variables in the bounds of other type variables

from typing import _T as T, TypeVar

L = TypeVar("L", bound=list[T])  # error: Type variable "typing._T" is unbound

def foo(l: L) -> L | T:  # error: A function returning TypeVar should receive at least one argument containing the same Typevar
    res = l[0]
    if res:
        return res
    return l

a: list[int] | list[str]

reveal_type(foo)  # "def [L <: list[T?], T] (l: L) -> L | T"
reveal_type(foo(a))  # list[int] | list[str]

Expected:

reveal_type(foo)  # "def [T, L <: list[T]] (l: L) -> L | T"
reveal_type(foo(a))  # list[int] | list[str] | int | str

Mypy should understand that T should be bound to foo from L

This is identical to generic TypeAliases:

L: TypeAlias = list[T]
a: L[int]

Here T is unbound, yet it's a valid and semantically sound expression.

Typescript example

The same idea could be represented in TypeScript as:

declare function foo<T, L extends T[]>(l: L): L | T

declare let a: number[] | string[]

let b = foo(a)

Although TS fails to infer the correct type here. (it infers as unknown)

basedmypy

This is partially supported in basedmypy

まず issue にある TypeVar とジェネリックな TypeAlias の例から始め、次に bounds と推論された型の現在の扱いを比較します。示されている reveal_type の出力が T を L 経由で制約されたものとして扱い、foo(a) に対して期待されるユニオンを生成すれば、作業は完了です。