Inconsistent processing Literal as TypeVar

Bug Report

Without an explicit cast to Literal mypy infers TypeVar as str (instead of Literal['a']) in the example below. But with an explicit cast, we have a redundant-cast error.

To Reproduce

import typing as tp

def foo[T](x: T) -> T: ...

a1 = foo('a')
a2 = foo(tp.cast(tp.Literal['a'], 'a'))
reveal_type(a1)  # Revealed type is "builtins.str"
reveal_type(a2)  # Revealed type is "Literal['a']"

https://mypy-play.net/?mypy=latest&python=3.12&flags=strict&gist=d58b88feb5b5ab8d53b3a566b15b5f03

It looks inconsistent, because if we run reveal_type('a') we'll get Literal['a']?, so why couldn't we use Literal['a'] as a return type? I didn't find this moment in typing spec or in the PEP, so probably, this behavior is not specified.

Other type checkers have different behavior (links for playground are given in brackets):

• pyright and pyrefly infer str in both cases;

• ty infers Literal.

Your Environment

• Mypy version used: 1.17.1
• Mypy command-line flags: --strict
• Mypy configuration options from mypy.ini (and other config files): -
• Python version used: 3.12

Inizia con il riproduttore Python 3.12 fornito usando --strict e confronta i risultati di reveal_type per le due chiamate. Traccia l’inferenza dei tipi nelle chiamate generiche e la gestione di Literal; il lavoro è completo quando il comportamento previsto è concordato e coperto da un test di regressione, incluso stabilire se il cast esplicito rimane ridondante.