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[ty] Better implementation of assignability for intersections with negated gradual elements (#20773)

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Alex Waygood 2025-10-10 12:10:17 +01:00 • committed by GitHub
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69
crates/ty_python_semantic/resources/mdtest/type_properties/is_assignable_to.md
View file

@ -721,6 +721,75 @@ static_assert(is_assignable_to(Intersection[LiteralString, Not[Literal[""]]], No
static_assert(is_assignable_to(Intersection[LiteralString, Not[Literal["", "a"]]], Not[AlwaysFalsy]))
```
## Intersections with non-fully-static negated elements
A type can be _assignable_ to an intersection containing negated elements only if the _bottom_
materialization of that type is disjoint from the _bottom_ materialization of all negated elements
in the intersection. This differs from subtyping, which should do the disjointness check against the
_top_ materialization of the negated elements.
```py
from typing_extensions import Any, Never, Sequence
from ty_extensions import Not, is_assignable_to, static_assert
# The bottom materialization of `tuple[Any]` is `tuple[Never]`,
# which simplifies to `Never`, so `tuple[int]` and `tuple[()]` are
# both assignable to `~tuple[Any]`
static_assert(is_assignable_to(tuple[int], Not[tuple[Any]]))
static_assert(is_assignable_to(tuple[()], Not[tuple[Any]]))
# But the bottom materialization of `tuple[Any, ...]` is `tuple[Never, ...]`,
# which simplifies to `tuple[()]`, so `tuple[int]` is still assignable to
# `~tuple[Any, ...]`, but `tuple[()]` is not
static_assert(is_assignable_to(tuple[int], Not[tuple[Any, ...]]))
static_assert(not is_assignable_to(tuple[()], Not[tuple[Any, ...]]))
# Similarly, the bottom materialization of `Sequence[Any]` is `Sequence[Never]`,
# so `tuple[()]` is not assignable to `~Sequence[Any]`, and nor is `list[Never]`,
# since both `tuple[()]` and `list[Never]` are subtypes of `Sequence[Never]`.
# `tuple[int, ...]` is also not assignable to `~Sequence[Any]`, as although it is
# not a subtype of `Sequence[Never]` it is also not disjoint from `Sequence[Never]`:
# `tuple[()]` is a subtype of both `Sequence[Never]` and `tuple[int, ...]`, so
# `tuple[int, ...]` and `Sequence[Never]` cannot be considered disjoint.
#
# Other `list` and `tuple` specializations *are* assignable to `~Sequence[Any]`,
# however, since there are many fully static materializations of `Sequence[Any]`
# that would be disjoint from a given `list` or `tuple` specialization.
static_assert(not is_assignable_to(tuple[()], Not[Sequence[Any]]))
static_assert(not is_assignable_to(list[Never], Not[Sequence[Any]]))
static_assert(not is_assignable_to(tuple[int, ...], Not[Sequence[Any]]))
# TODO: should pass (`tuple[int]` should be considered disjoint from `Sequence[Never]`)
static_assert(is_assignable_to(tuple[int], Not[Sequence[Any]])) # error: [static-assert-error]
# TODO: should pass (`list[int]` should be considered disjoint from `Sequence[Never]`)
static_assert(is_assignable_to(list[int], Not[Sequence[Any]])) # error: [static-assert-error]
# If the left-hand side is also not fully static,
# the left-hand side will be assignable to the right if the bottom materialization
# of the left-hand side is disjoint from the bottom materialization of all negated
# elements on the right-hand side
# `tuple[Any, ...]` cannot be assignable to `~tuple[Any, ...]`,
# because the bottom materialization of `tuple[Any, ...]` is
# `tuple[()]`, and `tuple[()]` is not disjoint from itself
static_assert(not is_assignable_to(tuple[Any, ...], Not[tuple[Any, ...]]))
# but `tuple[Any]` is assignable to `~tuple[Any]`,
# as the bottom materialization of `tuple[Any]` is `Never`,
# and `Never` *is* disjoint from itself
static_assert(is_assignable_to(tuple[Any], Not[tuple[Any]]))
# The same principle applies for non-fully-static `list` specializations.
# TODO: this should pass (`Bottom[list[Any]]` should simplify to `Never`)
static_assert(is_assignable_to(list[Any], Not[list[Any]])) # error: [static-assert-error]
# `Bottom[list[Any]]` is `Never`, which is disjoint from `Bottom[Sequence[Any]]`
# (which is `Sequence[Never]`).
# TODO: this should pass (`Bottom[list[Any]]` should simplify to `Never`)
static_assert(is_assignable_to(list[Any], Not[Sequence[Any]])) # error: [static-assert-error]
```
## General properties
See also: our property tests in `property_tests.rs`.

39
crates/ty_python_semantic/resources/mdtest/type_properties/is_subtype_of.md
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@ -533,6 +533,45 @@ static_assert(not is_subtype_of(int, Not[Literal[3]]))
static_assert(not is_subtype_of(Literal[1], Intersection[int, Not[Literal[1]]]))
```
## Intersections with non-fully-static negated elements
A type can be a _subtype_ of an intersection containing negated elements only if the _top_
materialization of that type is disjoint from the _top_ materialization of all negated elements in
the intersection. This differs from assignability, which should do the disjointness check against
the _bottom_ materialization of the negated elements.
```py
from typing_extensions import Any, Never, Sequence
from ty_extensions import Not, is_subtype_of, static_assert
# The top materialization of `tuple[Any]` is `tuple[object]`,
# which is disjoint from `tuple[()]` but not `tuple[int]`,
# so `tuple[()]` is a subtype of `~tuple[Any]` but `tuple[int]`
# is not.
static_assert(is_subtype_of(tuple[()], Not[tuple[Any]]))
static_assert(not is_subtype_of(tuple[int], Not[tuple[Any]]))
static_assert(not is_subtype_of(tuple[Any], Not[tuple[Any]]))
# The top materialization of `tuple[Any, ...]` is `tuple[object, ...]`,
# so no tuple type can be considered a subtype of `~tuple[Any, ...]`
static_assert(not is_subtype_of(tuple[()], Not[tuple[Any, ...]]))
static_assert(not is_subtype_of(tuple[int], Not[tuple[Any, ...]]))
static_assert(not is_subtype_of(tuple[int, ...], Not[tuple[Any, ...]]))
static_assert(not is_subtype_of(tuple[object, ...], Not[tuple[Any, ...]]))
static_assert(not is_subtype_of(tuple[Any, ...], Not[tuple[Any, ...]]))
# Similarly, the top materialization of `Sequence[Any]` is `Sequence[object]`,
# so no sequence type can be considered a subtype of `~Sequence[Any]`.
static_assert(not is_subtype_of(tuple[()], Not[Sequence[Any]]))
static_assert(not is_subtype_of(tuple[int], Not[Sequence[Any]]))
static_assert(not is_subtype_of(tuple[int, ...], Not[Sequence[Any]]))
static_assert(not is_subtype_of(tuple[object, ...], Not[Sequence[Any]]))
static_assert(not is_subtype_of(tuple[Any, ...], Not[Sequence[Any]]))
static_assert(not is_subtype_of(list[Never], Not[Sequence[Any]]))
static_assert(not is_subtype_of(list[Any], Not[Sequence[Any]]))
static_assert(not is_subtype_of(list[int], Not[Sequence[Any]]))
```
## Special types
### `Never`

40
crates/ty_python_semantic/src/types.rs
View file

@ -1757,8 +1757,29 @@ impl<'db> Type<'db> {
)
})
.and(db, || {
// For subtyping, we would want to check whether the *top materialization* of `self`
// is disjoint from the *top materialization* of `neg_ty`. As an optimization, however,
// we can avoid this explicit transformation here, since our `Type::is_disjoint_from`
// implementation already only returns true for `T.is_disjoint_from(U)` if the *top
// materialization* of `T` is disjoint from the *top materialization* of `U`.
//
// Note that the implementation of redundancy here may be too strict from a
// theoretical perspective: under redundancy, `T <: ~U` if `Bottom[T]` is disjoint
// from `Top[U]` and `Bottom[U]` is disjoint from `Top[T]`. It's possible that this
// could be improved. For now, however, we err on the side of strictness for our
// redundancy implementation: a fully complete implementation of redundancy may lead
// to non-transitivity (highly undesirable); and pragmatically, a full implementation
// of redundancy may not generally lead to simpler types in many situations.
let self_ty = match relation {
TypeRelation::Subtyping | TypeRelation::Redundancy => self,
TypeRelation::Assignability => self.bottom_materialization(db),
};
intersection.negative(db).iter().when_all(db, |&neg_ty| {
self.is_disjoint_from_impl(
let neg_ty = match relation {
TypeRelation::Subtyping | TypeRelation::Redundancy => neg_ty,
TypeRelation::Assignability => neg_ty.bottom_materialization(db),
};
self_ty.is_disjoint_from_impl(
db,
neg_ty,
disjointness_visitor,
@ -2284,10 +2305,21 @@ impl<'db> Type<'db> {
}
}
/// Return true if this type and `other` have no common elements.
/// Return true if `self & other` should simplify to `Never`:
/// if the intersection of the two types could never be inhabited by any
/// possible runtime value.
///
/// Note: This function aims to have no false positives, but might return
/// wrong `false` answers in some cases.
/// Our implementation of disjointness for non-fully-static types only
/// returns true if the *top materialization* of `self` has no overlap with
/// the *top materialization* of `other`.
///
/// For example, `list[int]` is disjoint from `list[str]`: the two types have
/// no overlap. But `list[Any]` is not disjoint from `list[str]`: there exists
/// a fully static materialization of `list[Any]` (`list[str]`) that is a
/// subtype of `list[str]`
///
/// This function aims to have no false positives, but might return wrong
/// `false` answers in some cases.
pub(crate) fn is_disjoint_from(self, db: &'db dyn Db, other: Type<'db>) -> bool {
self.when_disjoint_from(db, other).is_always_satisfied()
}