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

Start by reproducing the reported snippets in the mypy playground with mypy v1.14.1, no command-line flags, and compare the bound=object and bound=Any cases for type aliases and classes. Done means widening type variables produces consistent diagnostics at the same stage across these examples, matching the expected behavior described in the issue.