Fix narrowing for unions

[hauntsaninja]

Previously we considered the else branch unreachable in the testNarrowingAnyUnion test case. It's also nice that the new code is more obviously correct

Fixes #20330

This will help with landing #20727 as well

[hauntsaninja]

mesonbuild is desirable. the line is already type ignored because it is unsafe, now we get a better error message
starlette is a nice improvement, i will add a test for it
homeassistant huawei_lte is great, it makes mypy understand reachable code is reachable
homeassistant entity_registry is unfortunate. we infer dict[literal[...], any] where we used to infer dict[str, any]

[hauntsaninja]

This is #20330 . The regression meant we only accepted negative

[hauntsaninja]

This is essentially the starlette code. On master the else branch narrowing doesn't happen.

[hauntsaninja]

Not fully clear what this was testing. The change here is just that we simplify the union. I think the real thing this wants to test is that the else branch is unreachable, so I added that

[github-actions]

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

starlette (https://github.com/encode/starlette)
- starlette/middleware/errors.py:178: error: Argument 1 to "run_in_threadpool" has incompatible type "Callable[[Request[State], Exception], Response | Awaitable[Response]] | Callable[[WebSocket, Exception], Awaitable[None]]"; expected "Callable[[Request[State], Exception], Response]"  [arg-type]
+ starlette/middleware/errors.py:178: error: Argument 1 to "run_in_threadpool" has incompatible type "Callable[[Request[State], Exception], Response | Awaitable[Response]]"; expected "Callable[[Request[State], Exception], Response]"  [arg-type]
- starlette/_exception_handler.py:61: error: Argument 1 to "run_in_threadpool" has incompatible type "Callable[[Request[State], Exception], Response | Awaitable[Response]] | Callable[[WebSocket, Exception], Awaitable[None]]"; expected "Callable[[Request[State] | WebSocket, Exception], Any | None]"  [arg-type]
+ starlette/_exception_handler.py:61: error: Argument 2 to "run_in_threadpool" has incompatible type "Request[State] | WebSocket"; expected "Request[State]"  [arg-type]

core (https://github.com/home-assistant/core)
+ homeassistant/components/config/entity_registry.py:231: error: Invalid index type "Literal['aliases']" for "dict[Literal['device_class', 'disabled_by', 'hidden_by', 'icon', 'name', 'new_entity_id', 'area_id'], Any]"; expected type "Literal['device_class', 'disabled_by', 'hidden_by', 'icon', 'name', 'new_entity_id', 'area_id']"  [index]
+ homeassistant/components/config/entity_registry.py:235: error: Invalid index type "Literal['labels']" for "dict[Literal['device_class', 'disabled_by', 'hidden_by', 'icon', 'name', 'new_entity_id', 'area_id'], Any]"; expected type "Literal['device_class', 'disabled_by', 'hidden_by', 'icon', 'name', 'new_entity_id', 'area_id']"  [index]
+ homeassistant/components/config/entity_registry.py:260: error: Invalid index type "Literal['categories']" for "dict[Literal['device_class', 'disabled_by', 'hidden_by', 'icon', 'name', 'new_entity_id', 'area_id'], Any]"; expected type "Literal['device_class', 'disabled_by', 'hidden_by', 'icon', 'name', 'new_entity_id', 'area_id']"  [index]
- homeassistant/components/huawei_lte/__init__.py:192: error: Right operand of "and" is never evaluated  [unreachable]

meson (https://github.com/mesonbuild/meson)
+ mesonbuild/modules/rust.py:189: error: Unused "type: ignore" comment  [unused-ignore]
+ mesonbuild/modules/rust.py:189:39: error: TypedDict "ExecutableKeywordArguments" has no key "prelink"  [typeddict-item]
+ mesonbuild/modules/rust.py:189:39: note: Error code "typeddict-item" not covered by "type: ignore" comment
+ mesonbuild/modules/rust.py:189:39: error: TypedDict "ExecutableKeywordArguments" has no key "rust_abi"  [typeddict-item]
+ mesonbuild/modules/rust.py:189:39: error: TypedDict "ExecutableKeywordArguments" has no key "version"  [typeddict-item]
+ mesonbuild/modules/rust.py:189:39: error: TypedDict "ExecutableKeywordArguments" has no key "soversion"  [typeddict-item]
+ mesonbuild/modules/rust.py:189:39: error: TypedDict "ExecutableKeywordArguments" has no key "darwin_versions"  [typeddict-item]
+ mesonbuild/modules/rust.py:189:39: error: TypedDict "ExecutableKeywordArguments" has no key "shortname"  [typeddict-item]
+ mesonbuild/modules/rust.py:189:39: error: TypedDict "ExecutableKeywordArguments" has no key "pic"  [typeddict-item]