From 3c75a7705dfb10ffea76948337f30fa698539891 Mon Sep 17 00:00:00 2001
From: Carl Meyer <carl@astral.sh>
Date: Thu, 18 Sep 2025 15:23:02 -0700
Subject: [PATCH] salsa cache is_subtype_of, with cycle handling

---
 .../resources/mdtest/pep695_type_aliases.md   |  2 +-
 crates/ty_python_semantic/src/types.rs        | 20 +++++++++++++++++++
 2 files changed, 21 insertions(+), 1 deletion(-)

diff --git a/crates/ty_python_semantic/resources/mdtest/pep695_type_aliases.md b/crates/ty_python_semantic/resources/mdtest/pep695_type_aliases.md
index da915ac39b..ed4716494d 100644
--- a/crates/ty_python_semantic/resources/mdtest/pep695_type_aliases.md
+++ b/crates/ty_python_semantic/resources/mdtest/pep695_type_aliases.md
@@ -369,5 +369,5 @@ type Y = X | str | dict[str, Y]
 
 def _(y: Y):
     if isinstance(y, dict):
-        reveal_type(y)
+        reveal_type(y)  # revealed: dict[str, X] | dict[str, Y]
 ```
diff --git a/crates/ty_python_semantic/src/types.rs b/crates/ty_python_semantic/src/types.rs
index c2c11c5820..f8de6a507e 100644
--- a/crates/ty_python_semantic/src/types.rs
+++ b/crates/ty_python_semantic/src/types.rs
@@ -1423,6 +1423,7 @@ impl<'db> Type<'db> {
     /// P`, but not `B <: P`. Losing transitivity of subtyping is not tenable (it makes union and
     /// intersection simplification dependent on the order in which elements are added), so we do
     /// not use this more general definition of subtyping.
+    #[salsa::tracked(cycle_fn=is_subtype_of_cycle_recover, cycle_initial=is_subtype_of_cycle_initial)]
     pub(crate) fn is_subtype_of(self, db: &'db dyn Db, target: Type<'db>) -> bool {
         self.when_subtype_of(db, target).is_always_satisfied()
     }
@@ -6690,6 +6691,25 @@ impl<'db> VarianceInferable<'db> for Type<'db> {
     }
 }
 
+#[allow(clippy::trivially_copy_pass_by_ref)]
+fn is_subtype_of_cycle_recover<'db>(
+    _db: &'db dyn Db,
+    _value: &bool,
+    _count: u32,
+    _subtype: Type<'db>,
+    _supertype: Type<'db>,
+) -> salsa::CycleRecoveryAction<bool> {
+    salsa::CycleRecoveryAction::Iterate
+}
+
+fn is_subtype_of_cycle_initial<'db>(
+    _db: &'db dyn Db,
+    _subtype: Type<'db>,
+    _supertype: Type<'db>,
+) -> bool {
+    true
+}
+
 fn apply_specialization_cycle_recover<'db>(
     _db: &'db dyn Db,
     _value: &Type<'db>,