[ty] Preserve uncertain reflected operator dispatch

[charliermarsh]

Summary

This is a follow-up to #26434 that preserves both binary-operation dispatch results when static operand types do not determine their exact runtime classes.

For left: Base and right: Child, the runtime operands may be Base + Child, where Child.__radd__ has priority, or Child + Child, where Base.__add__ is called. We previously selected only the reflected result from the static subtype relationship.

This change classifies reflected priority as never, possible, or definite. Possible priority unions the normal and reflected call bindings; definite priority remains precise for exact left runtime classes such as the integer literal in 0 | Permission.READ. Bounded TypeVars and NewTypes use the same conservative distinction.

from typing import Literal

class Base:
    def __add__(self, other: object) -> Literal["base"]:
        return "base"

class Child(Base):
    def __radd__(self, other: object) -> Literal["child"]:
        return "child"

def add(left: Base, right: Child) -> Literal["base", "child"]:
    return left + right

[astral-sh-bot]

Typing conformance results

No changes detected ✅

Current numbers

The percentage of diagnostics emitted that were expected errors held steady at 94.47%. The percentage of expected errors that received a diagnostic held steady at 89.19%. The number of fully passing files held steady at 95/134.

[astral-sh-bot]

Memory usage report

Summary

Project	Old	New	Diff	Outcome
prefect	449.39MB	449.42MB	+0.01% (27.69kB)	⏫
sphinx	167.03MB	167.05MB	+0.01% (17.44kB)	⏫
trio	70.44MB	70.46MB	+0.02% (16.73kB)	⏫
flake8	29.02MB	29.02MB	+0.01% (2.21kB)	⏫

Significant changes

Click to expand detailed breakdown

prefect

Name	Old	New	Diff	Outcome
try_call_bin_op_return_type_impl	98.32kB	110.15kB	+12.03% (11.83kB)	⏫
member_lookup_with_policy_inner	10.23MB	10.23MB	+0.02% (2.37kB)	⏫
Type<'db>::class_member_with_policy_	7.92MB	7.92MB	+0.03% (2.36kB)	⏫
member_lookup_with_policy_inner::interned_arguments	6.74MB	6.74MB	+0.03% (1.88kB)	⏫
is_redundant_with_impl::interned_arguments	2.35MB	2.36MB	+0.07% (1.72kB)	⏫
Type<'db>::class_member_with_policy_::interned_arguments	5.74MB	5.74MB	+0.03% (1.62kB)	⏫
UnionType	1.07MB	1.07MB	+0.10% (1.09kB)	⏫
is_redundant_with_impl	1.31MB	1.31MB	+0.07% (944.00B)	⏫
UnionType<'db>::from_two_elements_::interned_arguments	270.62kB	271.48kB	+0.32% (880.00B)	⏫
infer_definition_types	50.03MB	50.04MB	+0.00% (608.00B)	⏫
UnionType<'db>::from_two_elements_	162.98kB	163.52kB	+0.33% (552.00B)	⏫
IntersectionType	985.54kB	986.03kB	+0.05% (504.00B)	⏫
OverloadLiteral	3.93MB	3.93MB	+0.01% (448.00B)	⏫
place_by_id	4.78MB	4.78MB	+0.01% (352.00B)	⏫
FunctionType	4.60MB	4.60MB	+0.01% (320.00B)	⏫
...			4 more

sphinx

Name	Old	New	Diff	Outcome
try_call_bin_op_return_type_impl	79.13kB	84.37kB	+6.61% (5.23kB)	⏫
Type<'db>::class_member_with_policy_	3.60MB	3.60MB	+0.05% (1.80kB)	⏫
member_lookup_with_policy_inner	4.29MB	4.29MB	+0.04% (1.62kB)	⏫
is_redundant_with_impl::interned_arguments	1.14MB	1.14MB	+0.13% (1.55kB)	⏫
member_lookup_with_policy_inner::interned_arguments	2.71MB	2.71MB	+0.05% (1.52kB)	⏫
Type<'db>::class_member_with_policy_::interned_arguments	2.36MB	2.36MB	+0.05% (1.32kB)	⏫
UnionType	583.61kB	584.59kB	+0.17% (1008.00B)	⏫
is_redundant_with_impl	631.71kB	632.55kB	+0.13% (864.00B)	⏫
UnionType<'db>::from_two_elements_::interned_arguments	174.80kB	175.57kB	+0.44% (792.00B)	⏫
IntersectionType	534.62kB	535.28kB	+0.12% (672.00B)	⏫
UnionType<'db>::from_two_elements_	101.38kB	101.88kB	+0.49% (504.00B)	⏫
place_by_id	1.33MB	1.33MB	+0.03% (352.00B)	⏫
place_by_id::interned_arguments	1.03MB	1.03MB	+0.03% (288.00B)	⏫
infer_scope_types_impl	8.10MB	8.10MB	+0.00% (16.00B)	⏫

trio

Name	Old	New	Diff	Outcome
member_lookup_with_policy_inner	1.02MB	1.03MB	+0.27% (2.81kB)	⏫
Type<'db>::class_member_with_policy_	887.43kB	890.16kB	+0.31% (2.73kB)	⏫
member_lookup_with_policy_inner::interned_arguments	764.77kB	766.99kB	+0.29% (2.23kB)	⏫
Type<'db>::class_member_with_policy_::interned_arguments	625.62kB	627.55kB	+0.31% (1.93kB)	⏫
try_call_bin_op_return_type_impl	17.91kB	19.08kB	+6.54% (1.17kB)	⏫
StaticClassLiteral<'db>::try_mro_	531.83kB	532.94kB	+0.21% (1.11kB)	⏫
is_redundant_with_impl::interned_arguments	237.02kB	237.96kB	+0.40% (968.00B)	⏫
UnionType	144.64kB	145.19kB	+0.38% (560.00B)	⏫
is_redundant_with_impl	129.38kB	129.89kB	+0.39% (520.00B)	⏫
IntersectionType	141.20kB	141.70kB	+0.35% (504.00B)	⏫
Specialization	464.61kB	465.05kB	+0.09% (448.00B)	⏫
UnionType<'db>::from_two_elements_::interned_arguments	29.48kB	29.91kB	+1.46% (440.00B)	⏫
StaticClassLiteral<'db>::try_mro_::interned_arguments	182.11kB	182.46kB	+0.19% (360.00B)	⏫
place_by_id	544.81kB	545.16kB	+0.06% (352.00B)	⏫
place_by_id::interned_arguments	413.09kB	413.37kB	+0.07% (288.00B)	⏫
...			7 more

flake8

Name	Old	New	Diff	Outcome
Type<'db>::class_member_with_policy_	236.45kB	236.86kB	+0.18% (424.00B)	⏫
member_lookup_with_policy_inner::interned_arguments	206.72kB	207.07kB	+0.17% (360.00B)	⏫
member_lookup_with_policy_inner	292.78kB	293.11kB	+0.11% (336.00B)	⏫
Type<'db>::class_member_with_policy_::interned_arguments	170.12kB	170.42kB	+0.18% (312.00B)	⏫
is_redundant_with_impl::interned_arguments	89.63kB	89.80kB	+0.19% (176.00B)	⏫
all_narrowing_constraints_for_expression	74.68kB	74.54kB	-0.19% (144.00B)	⏫
try_call_bin_op_return_type_impl	2.45kB	2.57kB	+4.78% (120.00B)	⏫
UnionType	57.66kB	57.77kB	+0.19% (112.00B)	⏫
OverloadLiteral	120.72kB	120.83kB	+0.09% (112.00B)	⏫
is_redundant_with_impl	51.77kB	51.86kB	+0.18% (96.00B)	⏫
infer_definition_types	1001.91kB	1001.99kB	+0.01% (88.00B)	⏫
UnionType<'db>::from_two_elements_::interned_arguments	10.23kB	10.31kB	+0.84% (88.00B)	⏫
place_by_id	135.58kB	135.66kB	+0.06% (88.00B)	⏫
infer_expression_types_impl	651.36kB	651.28kB	-0.01% (80.00B)	⏫
FunctionType	228.81kB	228.89kB	+0.03% (80.00B)	⏫
...			4 more

[AlexWaygood]

Not sure why the bot hasn't posted here, but note that there is an ecosystem diff here https://github.com/astral-sh/ruff/actions/runs/28457717664/attempts/1#summary-84337686112