Fix meet_types for literal and instance

[randolf-scholz]

Fixes #19560

• is_proper_subtype(Literal?, Literal) now always returns False; is_subtype retains original behavior.
• updated meet_types with special casing to carry over last_known_value correctly.

Currently, one unit test still fails, as an "unreachable" with match-case is not raises

[randolf-scholz]

The first operator failure seems like a bug in typeshed to me: python/typeshed#9004

[randolf-scholz]

Hm, the xarray redundant cast looks like this: mypy-playground

from typing import Literal, cast

type PDDatetimeUnitOptions = Literal["s", "ms", "ns"]

time_unit = cast(PDDatetimeUnitOptions, "s")  # [redundant-cast]

Is the current behavior of emitting redundant cast here intentional? It seems formally incorrect, since "s" should be Literal["s"]? and not Literal["s", "ms", "ns"], but maybe this was introduces as a convenience thing?

In comparison, pyright does not issue RedundantCast here, which aligns with my priors. pyright-playground

[github-actions]

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

prefect (https://github.com/PrefectHQ/prefect)
+ src/prefect/task_worker.py:370: error: Argument 1 to "submit" of "Executor" has incompatible type "Callable[[Callable[_P, _T], **_P], _T]"; expected "Callable[[Callable[[Task[P, R], Optional[UUID], Optional[TaskRun], Optional[dict[str, Any]], Union[PrefectFuture[Any], Any, Iterable[Union[PrefectFuture[Any], Any]], None], Literal['state', 'result'], Optional[dict[str, set[RunInput]]], Optional[dict[str, Any]]], Union[R, State[Any], None]], Task[[VarArg(Any), KwArg(Any)], Any], UUID, TaskRun, dict[Any, Any], list[Any], str, Optional[Any]], Union[Any, State[Any], None]]"  [arg-type]

pyinstrument (https://github.com/joerick/pyinstrument)
- pyinstrument/context_manager.py:40: error: Argument 1 to "Profiler" has incompatible type "**dict[str, Literal['enabled', 'disabled', 'strict'] | float | bool | None]"; expected "float"  [arg-type]
+ pyinstrument/context_manager.py:40: error: Argument 1 to "Profiler" has incompatible type "**dict[str, Literal['enabled', 'strict'] | float | str | bool | None]"; expected "float"  [arg-type]
- pyinstrument/context_manager.py:40: error: Argument 1 to "Profiler" has incompatible type "**dict[str, Literal['enabled', 'disabled', 'strict'] | float | bool | None]"; expected "Literal['enabled', 'disabled', 'strict']"  [arg-type]
+ pyinstrument/context_manager.py:40: error: Argument 1 to "Profiler" has incompatible type "**dict[str, Literal['enabled', 'strict'] | float | str | bool | None]"; expected "Literal['enabled', 'disabled', 'strict']"  [arg-type]
- pyinstrument/context_manager.py:40: error: Argument 1 to "Profiler" has incompatible type "**dict[str, Literal['enabled', 'disabled', 'strict'] | float | bool | None]"; expected "bool | None"  [arg-type]
+ pyinstrument/context_manager.py:40: error: Argument 1 to "Profiler" has incompatible type "**dict[str, Literal['enabled', 'strict'] | float | str | bool | None]"; expected "bool | None"  [arg-type]

operator (https://github.com/canonical/operator)
+ ops/model.py:831: error: Unsupported operand types for - ("set[tuple[Literal['tcp', 'udp', 'icmp'], int | None]]" and "set[tuple[str, int] | tuple[Literal['tcp', 'udp', 'icmp'], int | None]]")  [operator]
+ ops/model.py:833: error: Incompatible types in assignment (expression has type "str", variable has type "Literal['tcp', 'udp', 'icmp']")  [assignment]

strawberry (https://github.com/strawberry-graphql/strawberry)
- strawberry/codegen/query_codegen.py:476: error: Redundant cast to "Literal['query', 'mutation', 'subscription']"  [redundant-cast]

discord.py (https://github.com/Rapptz/discord.py)
- discord/app_commands/transformers.py:139: error: Incompatible types in assignment (expression has type "list[dict[str, Any]]", target has type "str | int")  [assignment]
+ discord/app_commands/transformers.py:139: error: Incompatible types in assignment (expression has type "list[dict[str, Any]]", target has type "bool | str | int")  [assignment]
- discord/app_commands/transformers.py:141: error: Incompatible types in assignment (expression has type "list[int]", target has type "str | int")  [assignment]
+ discord/app_commands/transformers.py:141: error: Incompatible types in assignment (expression has type "list[int]", target has type "bool | str | int")  [assignment]
- discord/app_commands/transformers.py:149: error: Incompatible types in assignment (expression has type "int | float", target has type "str | int")  [assignment]
+ discord/app_commands/transformers.py:149: error: Incompatible types in assignment (expression has type "int | float", target has type "bool | str | int")  [assignment]
- discord/app_commands/transformers.py:151: error: Incompatible types in assignment (expression has type "int | float", target has type "str | int")  [assignment]
+ discord/app_commands/transformers.py:151: error: Incompatible types in assignment (expression has type "int | float", target has type "bool | str | int")  [assignment]

dedupe (https://github.com/dedupeio/dedupe)
- dedupe/api.py:1547: error: Redundant cast to "Literal['match', 'distinct']"  [redundant-cast]

xarray (https://github.com/pydata/xarray)
- xarray/coding/times.py: note: In function "resolve_time_unit_from_attrs_dtype":
- xarray/coding/times.py:1420: error: Redundant cast to "Literal['s', 'ms', 'us', 'ns']"  [redundant-cast]
- xarray/coding/times.py:1433: error: Redundant cast to "Literal['s', 'ms', 'us', 'ns']"  [redundant-cast]
- xarray/coding/times.py: note: In member "decode" of class "CFTimedeltaCoder":
- xarray/coding/times.py:1537: error: Redundant cast to "Literal['s', 'ms', 'us', 'ns']"  [redundant-cast]