Treat widening type variables consistently

Bug Report

Currently, mypy allows widening type variables without an error, then errors (sometimes, see below) at usage time. This seems backwards to me. I'm opening this based on a minimization from @sterliakov in https://github.com/python/mypy/pull/18451.

To Reproduce

There's several interesting snippets:

import typing

IntT = typing.TypeVar("IntT", bound=int)
StrT = typing.TypeVar("StrT", bound=str)
AnyT = typing.TypeVar("AnyT", bound=typing.Any)

class IntC(typing.Generic[IntT]): ...
class StrC(typing.Generic[StrT]): ...

AnyC = IntC[AnyT] | StrC[AnyT]

def run(_: AnyC[StrT]) -> StrT: ...  # E: Type argument "StrT" of "IntC" must be a subtype of "int"

In this case, I would expect an error on the AnyC type alias definition, and maybe at usage time. What about other methods of widening type variables?

import typing

IntT = typing.TypeVar("IntT", bound=int)
StrT = typing.TypeVar("StrT", bound=str)
AnyT = typing.TypeVar("AnyT", bound=typing.Any)

class IntC(typing.Generic[IntT]): ...

class AnyC(IntC[AnyT]):  # ! no error
    pass

def f(x: StrT) -> StrT:
    y: AnyC[StrT]  # !? no error
    return x

This version doesn't error at all! What I would expect is the behavior achieved by using object plus a class to widen:

import typing

IntT = typing.TypeVar("IntT", bound=int)
StrT = typing.TypeVar("StrT", bound=str)
AnyT = typing.TypeVar("AnyT", bound=object)

class IntC(typing.Generic[IntT]): ...
class StrC(typing.Generic[StrT]): ...

class AnyC1(IntC[AnyT]):  # E: Type argument "AnyT" of "IntC" must be a subtype of "int"  
    pass

AnyC2 = IntC[AnyT] | StrC[AnyT]  # ! no error

def f(x: StrT) -> StrT:
    y1: AnyC1[StrT]  # no error, this is fine
    y2: AnyC2[StrT]  # E: Type argument "StrT" of "IntC" must be a subtype of "int"
    return x

Expected Behavior

Consistency.

Actual Behavior

To summarize the above

Your Environment

Everything was double checked in mypy playground.

• Mypy version used: v1.14.1
• Mypy command-line flags: none
• Mypy configuration options from mypy.ini (and other config files): N/A
• Python version used: 3.12

Inizia riproducendo gli snippet segnalati in mypy playground con mypy v1.14.1, senza flag da riga di comando, e confronta i casi bound=object e bound=Any per gli alias di tipo e le classi. Il lavoro è completato quando l’ampliamento delle variabili di tipo produce diagnostiche coerenti nella stessa fase in tutti questi esempi, in linea con il comportamento previsto descritto nell’issue.