salsa cache is_subtype_of, with cycle handling

Author: carljm

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]
 ```

crates/ty_python_semantic/src/types.rs

@@ -1422,6 +1422,7 @@ 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()
     }
@@ -6654,6 +6655,24 @@ impl<'db> VarianceInferable<'db> for Type<'db> {
     }
 }
 
+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>,