From beb8e2dfe07b71de5507fbff151e510fb284747a Mon Sep 17 00:00:00 2001
From: David Peter <sharkdp@users.noreply.github.com>
Date: Wed, 8 Jan 2025 20:25:08 +0100
Subject: [PATCH] [red-knot] More comprehensive `is_assignable_to` tests
 (#15353)

## Summary

This changeset migrates all existing `is_assignable_to` tests to a
Markdown-based test. It also increases our test coverage in a hopefully
meaningful way (not claiming to be complete in any sense). But at least
I found and fixed one bug while doing so.

## Test Plan

Ran property tests to make sure the new test succeeds after fixing it.

---------

Co-authored-by: Alex Waygood <Alex.Waygood@Gmail.com>
---
 .../type_properties/is_assignable_to.md       | 352 ++++++++++++++++++
 crates/red_knot_python_semantic/src/types.rs  |  83 +----
 .../src/types/property_tests.rs               |  14 +-
 3 files changed, 368 insertions(+), 81 deletions(-)
 create mode 100644 crates/red_knot_python_semantic/resources/mdtest/type_properties/is_assignable_to.md

diff --git a/crates/red_knot_python_semantic/resources/mdtest/type_properties/is_assignable_to.md b/crates/red_knot_python_semantic/resources/mdtest/type_properties/is_assignable_to.md
new file mode 100644
index 0000000000..e3f4187bb4
--- /dev/null
+++ b/crates/red_knot_python_semantic/resources/mdtest/type_properties/is_assignable_to.md
@@ -0,0 +1,352 @@
+# Assignable-to relation
+
+The `is_assignable_to(S, T)` relation below checks if type `S` is assignable to type `T` (target).
+This allows us to check if a type `S` can be used in a context where a type `T` is expected
+(function arguments, variable assignments). See the [typing documentation] for a precise definition
+of this concept.
+
+## Basic types
+
+### Fully static
+
+Fully static types participate in subtyping. If a type `S` is a subtype of `T`, `S` will also be
+assignable to `T`. Two equivalent types are subtypes of each other:
+
+```py
+from knot_extensions import static_assert, is_assignable_to
+
+class Parent: ...
+class Child1(Parent): ...
+class Child2(Parent): ...
+class Grandchild(Child1, Child2): ...
+class Unrelated: ...
+
+static_assert(is_assignable_to(int, int))
+static_assert(is_assignable_to(Parent, Parent))
+static_assert(is_assignable_to(Child1, Parent))
+static_assert(is_assignable_to(Grandchild, Parent))
+static_assert(is_assignable_to(Unrelated, Unrelated))
+
+static_assert(not is_assignable_to(str, int))
+static_assert(not is_assignable_to(object, int))
+static_assert(not is_assignable_to(Parent, Child1))
+static_assert(not is_assignable_to(Unrelated, Parent))
+static_assert(not is_assignable_to(Child1, Child2))
+```
+
+### Gradual types
+
+Gradual types do not participate in subtyping, but can still be assignable to other types (and
+static types can be assignable to gradual types):
+
+```py
+from knot_extensions import static_assert, is_assignable_to, Unknown
+from typing import Any
+
+static_assert(is_assignable_to(Unknown, Literal[1]))
+static_assert(is_assignable_to(Any, Literal[1]))
+static_assert(is_assignable_to(Literal[1], Unknown))
+static_assert(is_assignable_to(Literal[1], Any))
+```
+
+## Literal types
+
+### Boolean literals
+
+`Literal[True]` and `Literal[False]` are both subtypes of (and therefore assignable to) `bool`,
+which is in turn a subtype of `int`:
+
+```py
+from knot_extensions import static_assert, is_assignable_to
+from typing import Literal
+
+static_assert(is_assignable_to(Literal[True], Literal[True]))
+static_assert(is_assignable_to(Literal[True], bool))
+static_assert(is_assignable_to(Literal[True], int))
+
+static_assert(not is_assignable_to(Literal[True], Literal[False]))
+static_assert(not is_assignable_to(bool, Literal[True]))
+```
+
+### Integer literals
+
+```py
+from knot_extensions import static_assert, is_assignable_to
+from typing import Literal
+
+static_assert(is_assignable_to(Literal[1], Literal[1]))
+static_assert(is_assignable_to(Literal[1], int))
+
+static_assert(not is_assignable_to(Literal[1], Literal[2]))
+static_assert(not is_assignable_to(int, Literal[1]))
+static_assert(not is_assignable_to(Literal[1], str))
+```
+
+### String literals and `LiteralString`
+
+All string-literal types are subtypes of (and therefore assignable to) `LiteralString`, which is in
+turn a subtype of `str`:
+
+```py
+from knot_extensions import static_assert, is_assignable_to
+from typing_extensions import Literal, LiteralString
+
+static_assert(is_assignable_to(Literal["foo"], Literal["foo"]))
+static_assert(is_assignable_to(Literal["foo"], LiteralString))
+static_assert(is_assignable_to(Literal["foo"], str))
+
+static_assert(is_assignable_to(LiteralString, str))
+
+static_assert(not is_assignable_to(Literal["foo"], Literal["bar"]))
+static_assert(not is_assignable_to(str, Literal["foo"]))
+static_assert(not is_assignable_to(str, LiteralString))
+```
+
+### Byte literals
+
+```py
+from knot_extensions import static_assert, is_assignable_to
+from typing_extensions import Literal, LiteralString
+
+static_assert(is_assignable_to(Literal[b"foo"], bytes))
+static_assert(is_assignable_to(Literal[b"foo"], Literal[b"foo"]))
+
+static_assert(not is_assignable_to(Literal[b"foo"], str))
+static_assert(not is_assignable_to(Literal[b"foo"], LiteralString))
+static_assert(not is_assignable_to(Literal[b"foo"], Literal[b"bar"]))
+static_assert(not is_assignable_to(Literal[b"foo"], Literal["foo"]))
+static_assert(not is_assignable_to(Literal["foo"], Literal[b"foo"]))
+```
+
+## `type[…]` and class literals
+
+In the following tests, `TypeOf[str]` is a singleton type with a single inhabitant, the class `str`.
+This contrasts with `type[str]`, which represents "all possible subclasses of `str`".
+
+Both `TypeOf[str]` and `type[str]` are subtypes of `type` and `type[object]`, which both represent
+"all possible instances of `type`"; therefore both `type[str]` and `TypeOf[str]` are assignable to
+`type`. `type[Any]`, on the other hand, represents a type of unknown size or inhabitants, but which
+is known to be no larger than the set of possible objects represented by `type`.
+
+```py
+from knot_extensions import static_assert, is_assignable_to, Unknown, TypeOf
+from typing import Any
+
+static_assert(is_assignable_to(type, type))
+static_assert(is_assignable_to(type[object], type[object]))
+
+static_assert(is_assignable_to(type, type[object]))
+static_assert(is_assignable_to(type[object], type))
+
+static_assert(is_assignable_to(type[str], type[object]))
+static_assert(is_assignable_to(TypeOf[str], type[object]))
+static_assert(is_assignable_to(type[str], type))
+static_assert(is_assignable_to(TypeOf[str], type))
+
+static_assert(is_assignable_to(type[str], type[str]))
+static_assert(is_assignable_to(TypeOf[str], type[str]))
+
+static_assert(not is_assignable_to(TypeOf[int], type[str]))
+static_assert(not is_assignable_to(type, type[str]))
+static_assert(not is_assignable_to(type[object], type[str]))
+
+static_assert(is_assignable_to(type[Any], type[Any]))
+static_assert(is_assignable_to(type[Any], type[object]))
+static_assert(is_assignable_to(type[object], type[Any]))
+static_assert(is_assignable_to(type, type[Any]))
+static_assert(is_assignable_to(type[Any], type[str]))
+static_assert(is_assignable_to(type[str], type[Any]))
+static_assert(is_assignable_to(TypeOf[str], type[Any]))
+
+static_assert(is_assignable_to(type[Unknown], type[Unknown]))
+static_assert(is_assignable_to(type[Unknown], type[object]))
+static_assert(is_assignable_to(type[object], type[Unknown]))
+static_assert(is_assignable_to(type, type[Unknown]))
+static_assert(is_assignable_to(type[Unknown], type[str]))
+static_assert(is_assignable_to(type[str], type[Unknown]))
+static_assert(is_assignable_to(TypeOf[str], type[Unknown]))
+
+static_assert(is_assignable_to(type[Unknown], type[Any]))
+static_assert(is_assignable_to(type[Any], type[Unknown]))
+
+static_assert(not is_assignable_to(object, type[Any]))
+static_assert(not is_assignable_to(str, type[Any]))
+
+class Meta(type): ...
+
+static_assert(is_assignable_to(type[Any], Meta))
+static_assert(is_assignable_to(type[Unknown], Meta))
+static_assert(is_assignable_to(Meta, type[Any]))
+static_assert(is_assignable_to(Meta, type[Unknown]))
+```
+
+## Tuple types
+
+```py
+from knot_extensions import static_assert, is_assignable_to
+from typing import Literal, Any
+
+static_assert(is_assignable_to(tuple[()], tuple[()]))
+static_assert(is_assignable_to(tuple[int], tuple[int]))
+static_assert(is_assignable_to(tuple[int], tuple[Any]))
+static_assert(is_assignable_to(tuple[Any], tuple[int]))
+static_assert(is_assignable_to(tuple[int, str], tuple[int, str]))
+static_assert(is_assignable_to(tuple[Literal[1], Literal[2]], tuple[int, int]))
+static_assert(is_assignable_to(tuple[Any, Literal[2]], tuple[int, int]))
+static_assert(is_assignable_to(tuple[Literal[1], Any], tuple[int, int]))
+
+static_assert(not is_assignable_to(tuple[()], tuple[int]))
+static_assert(not is_assignable_to(tuple[int], tuple[str]))
+static_assert(not is_assignable_to(tuple[int], tuple[int, str]))
+static_assert(not is_assignable_to(tuple[int, str], tuple[int]))
+static_assert(not is_assignable_to(tuple[int, int], tuple[Literal[1], int]))
+static_assert(not is_assignable_to(tuple[Any, Literal[2]], tuple[int, str]))
+```
+
+## Union types
+
+```py
+from knot_extensions import static_assert, is_assignable_to, Unknown
+from typing import Literal, Any
+
+static_assert(is_assignable_to(int, int | str))
+static_assert(is_assignable_to(str, int | str))
+static_assert(is_assignable_to(int | str, int | str))
+static_assert(is_assignable_to(str | int, int | str))
+static_assert(is_assignable_to(Literal[1], int | str))
+static_assert(is_assignable_to(Literal[1], Unknown | str))
+static_assert(is_assignable_to(Literal[1] | Literal[2], Literal[1] | Literal[2]))
+static_assert(is_assignable_to(Literal[1] | Literal[2], int))
+static_assert(is_assignable_to(Literal[1] | None, int | None))
+static_assert(is_assignable_to(Any, int | str))
+static_assert(is_assignable_to(Any | int, int))
+static_assert(is_assignable_to(str, int | Any))
+
+static_assert(not is_assignable_to(int | None, int))
+static_assert(not is_assignable_to(int | None, str | None))
+static_assert(not is_assignable_to(Literal[1] | None, int))
+static_assert(not is_assignable_to(Literal[1] | None, str | None))
+static_assert(not is_assignable_to(Any | int | str, int))
+```
+
+## Intersection types
+
+```py
+from knot_extensions import static_assert, is_assignable_to, Intersection, Not
+from typing_extensions import Any, Literal
+
+class Parent: ...
+class Child1(Parent): ...
+class Child2(Parent): ...
+class Grandchild(Child1, Child2): ...
+class Unrelated: ...
+
+static_assert(is_assignable_to(Intersection[Child1, Child2], Child1))
+static_assert(is_assignable_to(Intersection[Child1, Child2], Child2))
+static_assert(is_assignable_to(Intersection[Child1, Child2], Parent))
+static_assert(is_assignable_to(Intersection[Child1, Parent], Parent))
+
+static_assert(is_assignable_to(Intersection[Parent, Unrelated], Parent))
+static_assert(is_assignable_to(Intersection[Child1, Unrelated], Child1))
+
+static_assert(is_assignable_to(Intersection[Child1, Not[Child2]], Child1))
+static_assert(is_assignable_to(Intersection[Child1, Not[Child2]], Parent))
+static_assert(is_assignable_to(Intersection[Child1, Not[Grandchild]], Parent))
+
+static_assert(is_assignable_to(Intersection[Child1, Child2], Intersection[Child1, Child2]))
+static_assert(is_assignable_to(Intersection[Child1, Child2], Intersection[Child2, Child1]))
+static_assert(is_assignable_to(Grandchild, Intersection[Child1, Child2]))
+
+static_assert(not is_assignable_to(Parent, Intersection[Parent, Unrelated]))
+static_assert(not is_assignable_to(int, Intersection[int, Not[Literal[1]]]))
+static_assert(not is_assignable_to(int, Not[int]))
+static_assert(not is_assignable_to(int, Not[Literal[1]]))
+
+static_assert(not is_assignable_to(Intersection[Any, Parent], Unrelated))
+
+# TODO: The following assertions should not fail (see https://github.com/astral-sh/ruff/issues/14899)
+# error: [static-assert-error]
+static_assert(is_assignable_to(Intersection[Any, int], int))
+
+# error: [static-assert-error]
+static_assert(is_assignable_to(Intersection[Unrelated, Any], Intersection[Unrelated, Any]))
+# error: [static-assert-error]
+static_assert(is_assignable_to(Intersection[Unrelated, Any], Intersection[Unrelated, Not[Any]]))
+# error: [static-assert-error]
+static_assert(is_assignable_to(Intersection[Unrelated, Any], Not[tuple[Unrelated, Any]]))
+```
+
+## General properties
+
+See also: our property tests in `property_tests.rs`.
+
+### Everything is assignable to `object`
+
+`object` is Python's top type; the set of all possible objects at runtime:
+
+```py
+from knot_extensions import static_assert, is_assignable_to, Unknown
+from typing import Literal, Any
+
+static_assert(is_assignable_to(str, object))
+static_assert(is_assignable_to(Literal[1], object))
+static_assert(is_assignable_to(object, object))
+static_assert(is_assignable_to(type, object))
+static_assert(is_assignable_to(Any, object))
+static_assert(is_assignable_to(Unknown, object))
+static_assert(is_assignable_to(type[object], object))
+static_assert(is_assignable_to(type[str], object))
+static_assert(is_assignable_to(type[Any], object))
+```
+
+### Every type is assignable to `Any` / `Unknown`
+
+`Any` and `Unknown` are gradual types. They could materialize to any given type at runtime, and so
+any type is assignable to them:
+
+```py
+from knot_extensions import static_assert, is_assignable_to, Unknown
+from typing import Literal, Any
+
+static_assert(is_assignable_to(str, Any))
+static_assert(is_assignable_to(Literal[1], Any))
+static_assert(is_assignable_to(object, Any))
+static_assert(is_assignable_to(type, Any))
+static_assert(is_assignable_to(Any, Any))
+static_assert(is_assignable_to(Unknown, Any))
+static_assert(is_assignable_to(type[object], Any))
+static_assert(is_assignable_to(type[str], Any))
+static_assert(is_assignable_to(type[Any], Any))
+
+static_assert(is_assignable_to(str, Unknown))
+static_assert(is_assignable_to(Literal[1], Unknown))
+static_assert(is_assignable_to(object, Unknown))
+static_assert(is_assignable_to(type, Unknown))
+static_assert(is_assignable_to(Any, Unknown))
+static_assert(is_assignable_to(Unknown, Unknown))
+static_assert(is_assignable_to(type[object], Unknown))
+static_assert(is_assignable_to(type[str], Unknown))
+static_assert(is_assignable_to(type[Any], Unknown))
+```
+
+### `Never` is assignable to every type
+
+`Never` is Python's bottom type: the empty set, a type with no inhabitants. It is therefore
+assignable to any arbitrary type.
+
+```py
+from knot_extensions import static_assert, is_assignable_to, Unknown
+from typing_extensions import Never, Any
+
+static_assert(is_assignable_to(Never, str))
+static_assert(is_assignable_to(Never, Literal[1]))
+static_assert(is_assignable_to(Never, object))
+static_assert(is_assignable_to(Never, type))
+static_assert(is_assignable_to(Never, Any))
+static_assert(is_assignable_to(Never, Unknown))
+static_assert(is_assignable_to(Never, type[object]))
+static_assert(is_assignable_to(Never, type[str]))
+static_assert(is_assignable_to(Never, type[Any]))
+```
+
+[typing documentation]: https://typing.readthedocs.io/en/latest/spec/concepts.html#the-assignable-to-or-consistent-subtyping-relation
diff --git a/crates/red_knot_python_semantic/src/types.rs b/crates/red_knot_python_semantic/src/types.rs
index 7f01d24e7a..f37dc5bf97 100644
--- a/crates/red_knot_python_semantic/src/types.rs
+++ b/crates/red_knot_python_semantic/src/types.rs
@@ -971,6 +971,9 @@ impl<'db> Type<'db> {
             return true;
         }
         match (self, target) {
+            // Never can be assigned to any type.
+            (Type::Never, _) => true,
+
             // The dynamic type is assignable-to and assignable-from any type.
             (Type::Unknown | Type::Any | Type::Todo(_), _) => true,
             (_, Type::Unknown | Type::Any | Type::Todo(_)) => true,
@@ -3988,7 +3991,6 @@ pub(crate) mod tests {
         },
         Tuple(Vec<Ty>),
         SubclassOfAny,
-        SubclassOfUnknown,
         SubclassOfBuiltinClass(&'static str),
         SubclassOfAbcClass(&'static str),
         StdlibModule(KnownModule),
@@ -4039,7 +4041,6 @@ pub(crate) mod tests {
                     TupleType::from_elements(db, elements)
                 }
                 Ty::SubclassOfAny => SubclassOfType::subclass_of_any(),
-                Ty::SubclassOfUnknown => SubclassOfType::subclass_of_unknown(),
                 Ty::SubclassOfBuiltinClass(s) => SubclassOfType::from(
                     db,
                     builtins_symbol(db, s)
@@ -4078,84 +4079,6 @@ pub(crate) mod tests {
         assert_eq!(ty.into_type(&db), Type::Never);
     }
 
-    #[test_case(Ty::BuiltinInstance("str"), Ty::BuiltinInstance("object"))]
-    #[test_case(Ty::BuiltinInstance("int"), Ty::BuiltinInstance("object"))]
-    #[test_case(Ty::Unknown, Ty::IntLiteral(1))]
-    #[test_case(Ty::Any, Ty::IntLiteral(1))]
-    #[test_case(Ty::Never, Ty::IntLiteral(1))]
-    #[test_case(Ty::IntLiteral(1), Ty::Unknown)]
-    #[test_case(Ty::IntLiteral(1), Ty::Any)]
-    #[test_case(Ty::IntLiteral(1), Ty::BuiltinInstance("int"))]
-    #[test_case(Ty::StringLiteral("foo"), Ty::BuiltinInstance("str"))]
-    #[test_case(Ty::StringLiteral("foo"), Ty::LiteralString)]
-    #[test_case(Ty::LiteralString, Ty::BuiltinInstance("str"))]
-    #[test_case(Ty::BytesLiteral("foo"), Ty::BuiltinInstance("bytes"))]
-    #[test_case(Ty::IntLiteral(1), Ty::Union(vec![Ty::BuiltinInstance("int"), Ty::BuiltinInstance("str")]))]
-    #[test_case(Ty::IntLiteral(1), Ty::Union(vec![Ty::Unknown, Ty::BuiltinInstance("str")]))]
-    #[test_case(Ty::Union(vec![Ty::IntLiteral(1), Ty::IntLiteral(2)]), Ty::Union(vec![Ty::IntLiteral(1), Ty::IntLiteral(2)]))]
-    #[test_case(
-        Ty::Union(vec![Ty::IntLiteral(1), Ty::IntLiteral(2)]),
-        Ty::BuiltinInstance("int")
-    )]
-    #[test_case(
-        Ty::Union(vec![Ty::IntLiteral(1), Ty::None]),
-        Ty::Union(vec![Ty::BuiltinInstance("int"), Ty::None])
-    )]
-    #[test_case(Ty::Tuple(vec![Ty::Todo]), Ty::Tuple(vec![Ty::IntLiteral(2)]))]
-    #[test_case(Ty::Tuple(vec![Ty::IntLiteral(2)]), Ty::Tuple(vec![Ty::Todo]))]
-    #[test_case(Ty::SubclassOfAny, Ty::SubclassOfAny)]
-    #[test_case(Ty::SubclassOfAny, Ty::SubclassOfBuiltinClass("object"))]
-    #[test_case(Ty::SubclassOfAny, Ty::SubclassOfBuiltinClass("str"))]
-    #[test_case(Ty::SubclassOfAny, Ty::BuiltinInstance("type"))]
-    #[test_case(Ty::SubclassOfBuiltinClass("object"), Ty::SubclassOfAny)]
-    #[test_case(
-        Ty::SubclassOfBuiltinClass("object"),
-        Ty::SubclassOfBuiltinClass("object")
-    )]
-    #[test_case(Ty::SubclassOfBuiltinClass("object"), Ty::BuiltinInstance("type"))]
-    #[test_case(Ty::SubclassOfBuiltinClass("str"), Ty::SubclassOfAny)]
-    #[test_case(
-        Ty::SubclassOfBuiltinClass("str"),
-        Ty::SubclassOfBuiltinClass("object")
-    )]
-    #[test_case(Ty::SubclassOfBuiltinClass("str"), Ty::SubclassOfBuiltinClass("str"))]
-    #[test_case(Ty::SubclassOfBuiltinClass("str"), Ty::BuiltinInstance("type"))]
-    #[test_case(Ty::BuiltinInstance("type"), Ty::SubclassOfAny)]
-    #[test_case(Ty::BuiltinInstance("type"), Ty::SubclassOfBuiltinClass("object"))]
-    #[test_case(Ty::BuiltinInstance("type"), Ty::BuiltinInstance("type"))]
-    #[test_case(Ty::BuiltinClassLiteral("str"), Ty::SubclassOfAny)]
-    #[test_case(Ty::SubclassOfBuiltinClass("str"), Ty::SubclassOfUnknown)]
-    #[test_case(Ty::SubclassOfUnknown, Ty::SubclassOfBuiltinClass("str"))]
-    #[test_case(Ty::SubclassOfAny, Ty::AbcInstance("ABCMeta"))]
-    #[test_case(Ty::SubclassOfUnknown, Ty::AbcInstance("ABCMeta"))]
-    #[test_case(Ty::SubclassOfAny, Ty::BuiltinInstance("object"))]
-    fn is_assignable_to(from: Ty, to: Ty) {
-        let db = setup_db();
-        assert!(from.into_type(&db).is_assignable_to(&db, to.into_type(&db)));
-    }
-
-    #[test_case(Ty::BuiltinInstance("object"), Ty::BuiltinInstance("int"))]
-    #[test_case(Ty::IntLiteral(1), Ty::BuiltinInstance("str"))]
-    #[test_case(Ty::BuiltinInstance("int"), Ty::BuiltinInstance("str"))]
-    #[test_case(Ty::BuiltinInstance("int"), Ty::IntLiteral(1))]
-    #[test_case(
-        Ty::Union(vec![Ty::IntLiteral(1), Ty::None]),
-        Ty::BuiltinInstance("int")
-    )]
-    #[test_case(
-        Ty::Union(vec![Ty::IntLiteral(1), Ty::None]),
-        Ty::Union(vec![Ty::BuiltinInstance("str"), Ty::None])
-    )]
-    #[test_case(
-        Ty::SubclassOfBuiltinClass("object"),
-        Ty::SubclassOfBuiltinClass("str")
-    )]
-    #[test_case(Ty::BuiltinInstance("type"), Ty::SubclassOfBuiltinClass("str"))]
-    fn is_not_assignable_to(from: Ty, to: Ty) {
-        let db = setup_db();
-        assert!(!from.into_type(&db).is_assignable_to(&db, to.into_type(&db)));
-    }
-
     #[test_case(Ty::BuiltinInstance("str"), Ty::BuiltinInstance("object"))]
     #[test_case(Ty::BuiltinInstance("int"), Ty::BuiltinInstance("object"))]
     #[test_case(Ty::BuiltinInstance("bool"), Ty::BuiltinInstance("object"))]
diff --git a/crates/red_knot_python_semantic/src/types/property_tests.rs b/crates/red_knot_python_semantic/src/types/property_tests.rs
index e18e8f15fe..93f9044909 100644
--- a/crates/red_knot_python_semantic/src/types/property_tests.rs
+++ b/crates/red_knot_python_semantic/src/types/property_tests.rs
@@ -220,7 +220,7 @@ macro_rules! type_property_test {
 }
 
 mod stable {
-    use super::KnownClass;
+    use crate::types::{KnownClass, Type};
 
     // `T` is equivalent to itself.
     type_property_test!(
@@ -299,6 +299,18 @@ mod stable {
         all_fully_static_types_subtype_of_object, db,
         forall types t. t.is_fully_static(db) => t.is_subtype_of(db, KnownClass::Object.to_instance(db))
     );
+
+    // Never should be assignable to every type
+    type_property_test!(
+        never_assignable_to_every_type, db,
+        forall types t. Type::Never.is_assignable_to(db, t)
+    );
+
+    // And it should be a subtype of all fully static types
+    type_property_test!(
+        never_subtype_of_every_fully_static_type, db,
+        forall types t. t.is_fully_static(db) => Type::Never.is_subtype_of(db, t)
+    );
 }
 
 /// This module contains property tests that currently lead to many false positives.