Extend special-case for context-based typevar inference in return position to typevar unions

[sterliakov]

• Fixes wrongly reported typing error in min with key lambda function and default value when result is Optional #17221.
• Fixes [arg-type] wrong argument type in generic function with union #17654.
• Fixes (🐞) generics not resolving correctly when multiple TypeVars don't meet the expected type #17553.
• Fixes min()/max() with default value cause issues with key function type #17536.
• Fixes Function returning union of type variables has inference/unification behaviour change when assigned to explicitly-typed location #16659.
• Fixes Type inference for max() "tainted" by default #16267.
• Fixes Inference failure in return statement with next and or #15755.
• Fixes itertools' all_equal recipe causes false arg-type failures #15150.
• Fixes False "Invalid index type" for min derived from return type #14664.
• Incidentally improves error message in Misleading warning for generic argument type on wrong return type #12156.
• Fixes Unneeded type widening of inner function call when using explicit return type #12092.
• Fixes Typechecking differs between one-line def of "return expr" vs. "r = expr; return r" #11985.
• Fixes generics inferred differently when assigning to variable even though the variable's type is the same #11455.
• Fixes False positive for lambda within a function #10426.

When using context, we can perform some overly optimistic inference when return type is T1 | T2. This breaks in important case of builtins.min when default and key are passed, essentially making them always incompatible. This is not the most principled approach, but let's see the primer results.

This resolves quite a few issues (some of them duplicates, but some - substantially different), min problem was a very popular one... Diff run: sterliakov/mypy-issues#30

This PR introduces one major regression. Namely, the following code becomes rejected:

from collections.abc import Mapping
from typing import TypeVar, Union

_T1 = TypeVar("_T1")

def check(mapping: Mapping[str, _T1]) -> None:
    fail1 = mapping.get("", "")
    fail2: Union[_T1, str] = mapping.get("", "")

Prior to this PR, fail1 was rejected but fail2 was accepted. Now both trigger a diagnostic. This problem stems from Mapping.get definition in typeshed:

class Mapping(Iterable[T], Generic[T, T_co], metaclass=ABCMeta):
    @overload
    def get(self, k: T) -> Optional[T_co]: pass
    @overload
    def get(self, k: T, default: Union[T_co, V]) -> Union[T_co, V]: pass
    ...

I don't know why union is used for default. This only applies to Mapping with this weird definition, dict overrides get to a longer version and works as intended.

class dict(MutableMapping[_KT, _VT]):
    # Positional-only in dict, but not in MutableMapping
    @overload  # type: ignore[override]
    def get(self, key: _KT, default: None = None, /) -> _VT | None: ...
    @overload
    def get(self, key: _KT, default: _VT, /) -> _VT: ...
    @overload
    def get(self, key: _KT, default: _T, /) -> _VT | _T: ...

I haven't yet figured out how to handle this sensibly assuming we don't want to change typeshed definition.

[github-actions]

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

antidote (https://github.com/Finistere/antidote)
- src/antidote/lib/interface_ext/_provider.py:313: error: Generator has incompatible item type "NewWeight"; expected "Weight"  [misc]
- src/antidote/lib/interface_ext/_provider.py:314: error: Argument 2 to "sum" has incompatible type "NewWeight"; expected "Weight"  [arg-type]
+ src/antidote/lib/interface_ext/_provider.py:312: error: Argument "weight" to "replace" of "CandidateImplementation[Weight]" has incompatible type "NewWeight"; expected "Weight"  [arg-type]

xarray (https://github.com/pydata/xarray)
+ xarray/core/utils.py: note: In function "get":
+ xarray/core/utils.py:503: error: Argument 2 to "get" of "Mapping" has incompatible type "T | None"; expected "V"  [arg-type]
+ xarray/conventions.py: note: In function "decode_cf_variables":
+ xarray/conventions.py:410: error: Argument "decode_times" to "decode_cf_variable" has incompatible type "object"; expected "bool | CFDatetimeCoder"  [arg-type]
+ xarray/conventions.py:413: error: Argument "decode_timedelta" to "decode_cf_variable" has incompatible type "object"; expected "bool | CFTimedeltaCoder | None"  [arg-type]

bidict (https://github.com/jab/bidict)
+ bidict/_base.py: note: In member "_dedup" of class "BidictBase":
+ bidict/_base.py:330:41: error: Argument 2 to "get" of "Mapping" has incompatible type "Literal[MissingT.MISSING]"; expected "VT"  [arg-type]
+ bidict/_base.py:331:41: error: Argument 2 to "get" of "Mapping" has incompatible type "Literal[MissingT.MISSING]"; expected "KT"  [arg-type]

Tanjun (https://github.com/FasterSpeeding/Tanjun)
- tanjun/clients.py:2428: error: Argument 1 to "get_type_dependency" of "Client" has incompatible type "type[SingleStoreCache[AuthorizationApplication]]"; expected "type[SingleStoreCache[Application]] | type[None]"  [arg-type]

operator (https://github.com/canonical/operator)
- ops/_main.py:95: error: Argument 2 to "next" has incompatible type "None"; expected "Storage"  [arg-type]
+ ops/_main.py:95: error: Incompatible types in assignment (expression has type "Storage | None", variable has type "Storage")  [assignment]

setuptools (https://github.com/pypa/setuptools)
+ setuptools/msvc.py:1442: error: Unused "type: ignore" comment  [unused-ignore]

werkzeug (https://github.com/pallets/werkzeug)
+ src/werkzeug/datastructures/mixins.py:280: error: Unused "type: ignore" comment  [unused-ignore]
+ src/werkzeug/datastructures/mixins.py:280: error: Incompatible types in assignment (expression has type "Union[V, T]", variable has type "V")  [assignment]
+ src/werkzeug/datastructures/mixins.py:280: note: Error code "assignment" not covered by "type: ignore" comment

pytest (https://github.com/pytest-dev/pytest)
+ src/_pytest/monkeypatch.py:293: error: Argument 2 to "get" of "Mapping" has incompatible type "Notset"; expected "V"  [arg-type]
+ src/_pytest/monkeypatch.py:307: error: Argument 2 to "get" of "Mapping" has incompatible type "Notset"; expected "V"  [arg-type]