From 6bdffc3cbff2194afdff0e46fccb03cf6147d367 Mon Sep 17 00:00:00 2001
From: Alex Waygood <Alex.Waygood@Gmail.com>
Date: Tue, 22 Apr 2025 15:14:10 +0100
Subject: [PATCH] [red-knot] Consider two instance types disjoint if the
 underlying classes have disjoint metaclasses (#17545)

---
 .../type_properties/is_disjoint_from.md       | 20 +++++++++++++--
 .../src/types/instance.rs                     | 25 +++++++++++++++++--
 2 files changed, 41 insertions(+), 4 deletions(-)

diff --git a/crates/red_knot_python_semantic/resources/mdtest/type_properties/is_disjoint_from.md b/crates/red_knot_python_semantic/resources/mdtest/type_properties/is_disjoint_from.md
index 173e643f3a..981851a6c7 100644
--- a/crates/red_knot_python_semantic/resources/mdtest/type_properties/is_disjoint_from.md
+++ b/crates/red_knot_python_semantic/resources/mdtest/type_properties/is_disjoint_from.md
@@ -56,6 +56,19 @@ static_assert(not is_disjoint_from(FinalSubclass, A))
 # ... which makes it disjoint from B1, B2:
 static_assert(is_disjoint_from(B1, FinalSubclass))
 static_assert(is_disjoint_from(B2, FinalSubclass))
+
+# Instance types can also be disjoint if they have disjoint metaclasses.
+# No possible subclass of `Meta1` and `Meta2` could exist, therefore
+# no possible subclass of `UsesMeta1` and `UsesMeta2` can exist:
+class Meta1(type): ...
+class UsesMeta1(metaclass=Meta1): ...
+
+@final
+class Meta2(type): ...
+
+class UsesMeta2(metaclass=Meta2): ...
+
+static_assert(is_disjoint_from(UsesMeta1, UsesMeta2))
 ```
 
 ## Tuple types
@@ -342,8 +355,8 @@ static_assert(is_disjoint_from(Meta1, type[UsesMeta2]))
 
 ### `type[T]` versus `type[S]`
 
-By the same token, `type[T]` is disjoint from `type[S]` if the metaclass of `T` is disjoint from the
-metaclass of `S`.
+By the same token, `type[T]` is disjoint from `type[S]` if `T` is `@final`, `S` is `@final`, or the
+metaclass of `T` is disjoint from the metaclass of `S`.
 
 ```py
 from typing import final
@@ -353,6 +366,9 @@ from knot_extensions import static_assert, is_disjoint_from
 class Meta1(type): ...
 
 class Meta2(type): ...
+
+static_assert(is_disjoint_from(type[Meta1], type[Meta2]))
+
 class UsesMeta1(metaclass=Meta1): ...
 class UsesMeta2(metaclass=Meta2): ...
 
diff --git a/crates/red_knot_python_semantic/src/types/instance.rs b/crates/red_knot_python_semantic/src/types/instance.rs
index 1b0ec2f4fd..fb1b1deb76 100644
--- a/crates/red_knot_python_semantic/src/types/instance.rs
+++ b/crates/red_knot_python_semantic/src/types/instance.rs
@@ -43,8 +43,29 @@ impl<'db> InstanceType<'db> {
     }
 
     pub(super) fn is_disjoint_from(self, db: &'db dyn Db, other: Self) -> bool {
-        (self.class.is_final(db) && !self.class.is_subclass_of(db, other.class))
-            || (other.class.is_final(db) && !other.class.is_subclass_of(db, self.class))
+        if self.class.is_final(db) && !self.class.is_subclass_of(db, other.class) {
+            return true;
+        }
+
+        if other.class.is_final(db) && !other.class.is_subclass_of(db, self.class) {
+            return true;
+        }
+
+        // Check to see whether the metaclasses of `self` and `other` are disjoint.
+        // Avoid this check if the metaclass of either `self` or `other` is `type`,
+        // however, since we end up with infinite recursion in that case due to the fact
+        // that `type` is its own metaclass (and we know that `type` cannot be disjoint
+        // from any metaclass, anyway).
+        let type_type = KnownClass::Type.to_instance(db);
+        let self_metaclass = self.class.metaclass_instance_type(db);
+        if self_metaclass == type_type {
+            return false;
+        }
+        let other_metaclass = other.class.metaclass_instance_type(db);
+        if other_metaclass == type_type {
+            return false;
+        }
+        self_metaclass.is_disjoint_from(db, other_metaclass)
     }
 
     pub(super) fn is_gradual_equivalent_to(self, db: &'db dyn Db, other: Self) -> bool {