Fix missing meet case exposed by len narrowing

[ilevkivskyi]

Fixes #16468

The fix is straightforward.

Btw when fixing this I noticed that we disregard type arguments when narrowing, for example:

x: Sequence[int]
if isinstance(x, tuple):
    reveal_type(x)  # tuple[Any, ...], but should be `tuple[int, ...]`

I guess fixing this may be tricky, and it is quite old behavior.

[github-actions]

Diff from mypy_primer, showing the effect of this PR on open source code:

websockets (https://github.com/aaugustin/websockets)
+ src/websockets/legacy/auth.py:162: error: Argument 1 to "list" has incompatible type "tuple[str, str] | Iterable[tuple[str, str]]"; expected "Iterable[tuple[str, str]]"  [arg-type]

vision (https://github.com/pytorch/vision)
+ torchvision/transforms/v2/_geometry.py:254: error: Redundant cast to "tuple[float, float]"  [redundant-cast]
+ torchvision/transforms/v2/_geometry.py:257: error: Redundant cast to "tuple[float, float]"  [redundant-cast]

[ilevkivskyi]

New error in websockets is kind of correct. The code should use a TypeGuard to prevent single tuple from getting into second elif (because tuple is also iterable).

[sobolevn]

Maybe add else: reveal_type(x)? It might give some unexpected result.

[ilevkivskyi]

The else branch reveals Union[builtins.int, typing.Sequence[builtins.int], builtins.tuple[Any, ...]] because of the behavior I mentioned in the description. I intentionally didn't add this, since I don't want to create an impression this is intentional.