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

Beginne damit, die gemeldeten Snippets im mypy playground mit mypy v1.14.1 ohne Kommandozeilen-Flags zu reproduzieren und die Fälle bound=object und bound=Any für Typ-Aliase und Klassen zu vergleichen. Als abgeschlossen gilt die Aufgabe, wenn das Erweitern von Typvariablen in diesen Beispielen konsistente Diagnosen in derselben Phase erzeugt und dem im Issue beschriebenen erwarteten Verhalten entspricht.