[red-knot] Simplify object out of intersections (#15511)

Author: AlexWaygood

crates/red_knot_python_semantic/resources/mdtest/comparison/intersections.md

@@ -92,8 +92,7 @@ def _(o: object):
     n = None
 
     if o is not None:
-        reveal_type(o)  # revealed: object & ~None
-
+        reveal_type(o)  # revealed:  ~None
         reveal_type(o is n)  # revealed: Literal[False]
         reveal_type(o is not n)  # revealed: Literal[True]
 ```

crates/red_knot_python_semantic/resources/mdtest/intersection_types.md

@@ -283,6 +283,21 @@ def _(
     reveal_type(not_object)  # revealed: Never
 ```
 
+### `object & ~T` is equivalent to `~T`
+
+A second consequence of the fact that `object` is the top type is that `object` is always redundant
+in intersections, and can be eagerly simplified out. `object & P` is equivalent to `P`;
+`object & ~P` is equivalent to `~P` for any type `P`.
+
+```py
+from knot_extensions import Intersection, Not, is_equivalent_to, static_assert
+
+class P: ...
+
+static_assert(is_equivalent_to(Intersection[object, P], P))
+static_assert(is_equivalent_to(Intersection[object, Not[P]], Not[P]))
+```
+
 ### Intersection of a type and its negation
 
 Continuing with more [complement laws], if we see both `P` and `~P` in an intersection, we can

crates/red_knot_python_semantic/resources/mdtest/narrow/isinstance.md

@@ -91,8 +91,7 @@ if isinstance(x, (A, B)):
 elif isinstance(x, (A, C)):
     reveal_type(x)  # revealed: C & ~A & ~B
 else:
-    # TODO: Should be simplified to ~A & ~B & ~C
-    reveal_type(x)  # revealed: object & ~A & ~B & ~C
+    reveal_type(x)  # revealed: ~A & ~B & ~C
 ```
 
 ## No narrowing for instances of `builtins.type`

crates/red_knot_python_semantic/src/types/builder.rs

@@ -284,10 +284,16 @@ impl<'db> InnerIntersectionBuilder<'db> {
                 }
             }
             _ => {
-                let addition_is_bool_instance = new_positive
+                let known_instance = new_positive
                     .into_instance()
-                    .and_then(|instance| instance.class.known(db))
-                    .is_some_and(KnownClass::is_bool);
+                    .and_then(|instance| instance.class.known(db));
+
+                if known_instance == Some(KnownClass::Object) {
+                    // `object & T` -> `T`; it is always redundant to add `object` to an intersection
+                    return;
+                }
+
+                let addition_is_bool_instance = known_instance == Some(KnownClass::Bool);
 
                 for (index, existing_positive) in self.positive.iter().enumerate() {
                     match existing_positive {