[red-knot] T | object == object (#16088)

## Summary

- Simplify unions with `object` to `object`.
- Add a new `Type::object(db)` constructor to abbreviate
`KnownClass::Object.to_instance(db)` in some places.
- Add a `Type::is_object` and `Class::is_object` function to make some
tests for a bit easier to read.

closes #16084

## Test Plan

New Markdown tests.

crates/red_knot_python_semantic/resources/mdtest/union_types.md

@@ -37,6 +37,31 @@ def noreturn(u1: int | NoReturn, u2: int | NoReturn | str) -> None:
     reveal_type(u2)  # revealed:  int | str
 ```
 
+## `object` subsumes everything
+
+Unions with `object` can be simplified to `object`:
+
+```py
+from typing_extensions import Never, Any
+
+def _(
+    u1: int | object,
+    u2: object | int,
+    u3: Any | object,
+    u4: object | Any,
+    u5: object | Never,
+    u6: Never | object,
+    u7: int | str | object | bytes | Any,
+) -> None:
+    reveal_type(u1)  # revealed: object
+    reveal_type(u2)  # revealed: object
+    reveal_type(u3)  # revealed: object
+    reveal_type(u4)  # revealed: object
+    reveal_type(u5)  # revealed: object
+    reveal_type(u6)  # revealed: object
+    reveal_type(u7)  # revealed: object
+```
+
 ## Flattening of nested unions
 
 ```py
@@ -120,8 +145,8 @@ Simplifications still apply when `Unknown` is present.
 ```py
 from knot_extensions import Unknown
 
-def _(u1: str | Unknown | int | object):
-    reveal_type(u1)  # revealed: Unknown | object
+def _(u1: int | Unknown | bool) -> None:
+    reveal_type(u1)  # revealed: int | Unknown
 ```
 
 ## Union of intersections

crates/red_knot_python_semantic/src/types.rs

@@ -631,6 +631,10 @@ impl<'db> Type<'db> {
         Self::Dynamic(DynamicType::Unknown)
     }
 
+    pub fn object(db: &'db dyn Db) -> Self {
+        KnownClass::Object.to_instance(db)
+    }
+
     pub const fn is_unknown(&self) -> bool {
         matches!(self, Type::Dynamic(DynamicType::Unknown))
     }
@@ -639,6 +643,11 @@ impl<'db> Type<'db> {
         matches!(self, Type::Never)
     }
 
+    pub fn is_object(&self, db: &'db dyn Db) -> bool {
+        self.into_instance()
+            .is_some_and(|instance| instance.class.is_object(db))
+    }
+
     pub const fn is_todo(&self) -> bool {
         matches!(self, Type::Dynamic(DynamicType::Todo(_)))
     }
@@ -895,7 +904,7 @@ impl<'db> Type<'db> {
             // `object` is the only type that can be known to be a supertype of any intersection,
             // even an intersection with no positive elements
             (Type::Intersection(_), Type::Instance(InstanceType { class }))
-                if class.is_known(db, KnownClass::Object) =>
+                if class.is_object(db) =>
             {
                 true
             }
@@ -949,7 +958,7 @@ impl<'db> Type<'db> {
             (left, Type::AlwaysTruthy) => left.bool(db).is_always_true(),
             // Currently, the only supertype of `AlwaysFalsy` and `AlwaysTruthy` is the universal set (object instance).
             (Type::AlwaysFalsy | Type::AlwaysTruthy, _) => {
-                target.is_equivalent_to(db, KnownClass::Object.to_instance(db))
+                target.is_equivalent_to(db, Type::object(db))
             }
 
             // All `StringLiteral` types are a subtype of `LiteralString`.
@@ -1088,11 +1097,7 @@ impl<'db> Type<'db> {
 
             // All types are assignable to `object`.
             // TODO this special case might be removable once the below cases are comprehensive
-            (_, Type::Instance(InstanceType { class }))
-                if class.is_known(db, KnownClass::Object) =>
-            {
-                true
-            }
+            (_, Type::Instance(InstanceType { class })) if class.is_object(db) => true,
 
             // A union is assignable to a type T iff every element of the union is assignable to T.
             (Type::Union(union), ty) => union
@@ -1801,7 +1806,7 @@ impl<'db> Type<'db> {
                     // TODO should be `Callable[[], Literal[True/False]]`
                     todo_type!("`__bool__` for `AlwaysTruthy`/`AlwaysFalsy` Type variants").into()
                 }
-                _ => KnownClass::Object.to_instance(db).member(db, name),
+                _ => Type::object(db).member(db, name),
             },
         }
     }
@@ -3853,6 +3858,11 @@ impl<'db> Class<'db> {
         self.known(db) == Some(known_class)
     }
 
+    /// Return `true` if this class represents the builtin class `object`
+    pub fn is_object(self, db: &'db dyn Db) -> bool {
+        self.is_known(db, KnownClass::Object)
+    }
+
     /// Return an iterator over the inferred types of this class's *explicit* bases.
     ///
     /// Note that any class (except for `object`) that has no explicit

crates/red_knot_python_semantic/src/types/builder.rs

@@ -43,6 +43,13 @@ impl<'db> UnionBuilder<'db> {
         }
     }
 
+    /// Collapse the union to a single type: `object`.
+    fn collapse_to_object(mut self) -> Self {
+        self.elements.clear();
+        self.elements.push(Type::object(self.db));
+        self
+    }
+
     /// Adds a type to this union.
     pub(crate) fn add(mut self, ty: Type<'db>) -> Self {
         match ty {
@@ -53,7 +60,12 @@ impl<'db> UnionBuilder<'db> {
                     self = self.add(*element);
                 }
             }
+            // Adding `Never` to a union is a no-op.
             Type::Never => {}
+            // Adding `object` to a union results in `object`.
+            ty if ty.is_object(self.db) => {
+                return self.collapse_to_object();
+            }
             _ => {
                 let bool_pair = if let Type::BooleanLiteral(b) = ty {
                     Some(Type::BooleanLiteral(!b))
@@ -76,7 +88,10 @@ impl<'db> UnionBuilder<'db> {
                         break;
                     }
 
-                    if ty.is_same_gradual_form(*element) || ty.is_subtype_of(self.db, *element) {
+                    if ty.is_same_gradual_form(*element)
+                        || ty.is_subtype_of(self.db, *element)
+                        || element.is_object(self.db)
+                    {
                         return self;
                     } else if element.is_subtype_of(self.db, ty) {
                         to_remove.push(index);
@@ -88,9 +103,7 @@ impl<'db> UnionBuilder<'db> {
                         // `element | ty` must be `object` (object has no other supertypes). This means we can simplify
                         // the whole union to just `object`, since all other potential elements would also be subtypes of
                         // `object`.
-                        self.elements.clear();
-                        self.elements.push(KnownClass::Object.to_instance(self.db));
-                        return self;
+                        return self.collapse_to_object();
                     }
                 }
                 match to_remove[..] {
@@ -416,7 +429,7 @@ impl<'db> InnerIntersectionBuilder<'db> {
             Type::Never => {
                 // Adding ~Never to an intersection is a no-op.
             }
-            Type::Instance(instance) if instance.class.is_known(db, KnownClass::Object) => {
+            Type::Instance(instance) if instance.class.is_object(db) => {
                 // Adding ~object to an intersection results in Never.
                 *self = Self::default();
                 self.positive.insert(Type::Never);
@@ -481,7 +494,7 @@ impl<'db> InnerIntersectionBuilder<'db> {
 
     fn build(mut self, db: &'db dyn Db) -> Type<'db> {
         match (self.positive.len(), self.negative.len()) {
-            (0, 0) => KnownClass::Object.to_instance(db),
+            (0, 0) => Type::object(db),
             (1, 0) => self.positive[0],
             _ => {
                 self.positive.shrink_to_fit();
@@ -534,7 +547,7 @@ mod tests {
         let db = setup_db();
 
         let intersection = IntersectionBuilder::new(&db).build();
-        assert_eq!(intersection, KnownClass::Object.to_instance(&db));
+        assert_eq!(intersection, Type::object(&db));
     }
 
     #[test_case(Type::BooleanLiteral(true))]
@@ -548,7 +561,7 @@ mod tests {
         // We add t_object in various orders (in first or second position) in
         // the tests below to ensure that the boolean simplification eliminates
         // everything from the intersection, not just `bool`.
-        let t_object = KnownClass::Object.to_instance(&db);
+        let t_object = Type::object(&db);
         let t_bool = KnownClass::Bool.to_instance(&db);
 
         let ty = IntersectionBuilder::new(&db)

crates/red_knot_python_semantic/src/types/mro.rs

@@ -4,7 +4,7 @@ use std::ops::Deref;
 use rustc_hash::FxHashSet;
 
 use crate::types::class_base::ClassBase;
-use crate::types::{Class, KnownClass, Type};
+use crate::types::{Class, Type};
 use crate::Db;
 
 /// The inferred method resolution order of a given class.
@@ -52,9 +52,7 @@ impl<'db> Mro<'db> {
         match class_bases {
             // `builtins.object` is the special case:
             // the only class in Python that has an MRO with length <2
-            [] if class.is_known(db, KnownClass::Object) => {
-                Ok(Self::from([ClassBase::Class(class)]))
-            }
+            [] if class.is_object(db) => Ok(Self::from([ClassBase::Class(class)])),
 
             // All other classes in Python have an MRO with length >=2.
             // Even if a class has no explicit base classes,

crates/red_knot_python_semantic/src/types/narrow.rs

@@ -152,13 +152,13 @@ fn merge_constraints_or<'db>(
                 *entry.get_mut() = UnionBuilder::new(db).add(*entry.get()).add(*value).build();
             }
             Entry::Vacant(entry) => {
-                entry.insert(KnownClass::Object.to_instance(db));
+                entry.insert(Type::object(db));
             }
         }
     }
     for (key, value) in into.iter_mut() {
         if !from.contains_key(key) {
-            *value = KnownClass::Object.to_instance(db);
+            *value = Type::object(db);
         }
     }
 }

crates/red_knot_python_semantic/src/types/property_tests.rs

@@ -329,7 +329,7 @@ fn union<'db>(db: &'db TestDb, tys: impl IntoIterator<Item = Type<'db>>) -> Type
 
 mod stable {
     use super::union;
-    use crate::types::{KnownClass, Type};
+    use crate::types::Type;
 
     // Reflexivity: `T` is equivalent to itself.
     type_property_test!(
@@ -419,13 +419,13 @@ mod stable {
     // All types should be assignable to `object`
     type_property_test!(
         all_types_assignable_to_object, db,
-        forall types t. t.is_assignable_to(db, KnownClass::Object.to_instance(db))
+        forall types t. t.is_assignable_to(db, Type::object(db))
     );
 
     // And for fully static types, they should also be subtypes of `object`
     type_property_test!(
         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))
+        forall types t. t.is_fully_static(db) => t.is_subtype_of(db, Type::object(db))
     );
 
     // Never should be assignable to every type

crates/red_knot_python_semantic/src/types/signatures.rs

@@ -411,7 +411,7 @@ mod tests {
                 },
                 Parameter {
                     name: Some(Name::new_static("args")),
-                    annotated_ty: Some(KnownClass::Object.to_instance(&db)),
+                    annotated_ty: Some(Type::object(&db)),
                     kind: ParameterKind::Variadic,
                 },
                 Parameter {

crates/red_knot_python_semantic/src/types/subclass_of.rs

@@ -26,7 +26,7 @@ impl<'db> SubclassOfType<'db> {
             ClassBase::Class(class) => {
                 if class.is_final(db) {
                     Type::ClassLiteral(ClassLiteralType { class })
-                } else if class.is_known(db, KnownClass::Object) {
+                } else if class.is_object(db) {
                     KnownClass::Type.to_instance(db)
                 } else {
                     Type::SubclassOf(Self { subclass_of })