[ty] Fix panic on recursive class definitions in a stub that use constrained type variables (#20955)

crates/ty_python_semantic/resources/mdtest/generics/legacy/classes.md

@@ -753,5 +753,29 @@ class C(C, Generic[T]): ...
 class D(D[int], Generic[T]): ...
 ```
 
+### Cyclic inheritance in a stub file combined with constrained type variables
+
+This is a regression test for <https://github.com/astral-sh/ty/issues/1390>; we used to panic on
+this:
+
+`stub.pyi`:
+
+```pyi
+from typing import Generic, TypeVar
+
+class A(B): ...
+class G: ...
+
+T = TypeVar("T", G, A)
+
+class C(Generic[T]): ...
+class B(C[A]): ...
+class D(C[G]): ...
+
+def func(x: D): ...
+
+func(G())  # error: [invalid-argument-type]
+```
+
 [crtp]: https://en.wikipedia.org/wiki/Curiously_recurring_template_pattern
 [f-bound]: https://en.wikipedia.org/wiki/Bounded_quantification#F-bounded_quantification

crates/ty_python_semantic/resources/mdtest/generics/pep695/classes.md

@@ -638,6 +638,25 @@ class C[T](C): ...
 class D[T](D[int]): ...
 ```
 
+### Cyclic inheritance in a stub file combined with constrained type variables
+
+This is a regression test for <https://github.com/astral-sh/ty/issues/1390>; we used to panic on
+this:
+
+`stub.pyi`:
+
+```pyi
+class A(B): ...
+class G: ...
+class C[T: (G, A)]: ...
+class B(C[A]): ...
+class D(C[G]): ...
+
+def func(x: D): ...
+
+func(G())  # error: [invalid-argument-type]
+```
+
 ## Tuple as a PEP-695 generic class
 
 Our special handling for `tuple` does not break if `tuple` is defined as a PEP-695 generic class in

crates/ty_python_semantic/src/types.rs

@@ -8303,7 +8303,11 @@ impl<'db> TypeVarInstance<'db> {
         ))
     }
 
-    #[salsa::tracked(cycle_fn=lazy_bound_cycle_recover, cycle_initial=lazy_bound_cycle_initial, heap_size=ruff_memory_usage::heap_size)]
+    #[salsa::tracked(
+        cycle_fn=lazy_bound_or_constraints_cycle_recover,
+        cycle_initial=lazy_bound_or_constraints_cycle_initial,
+        heap_size=ruff_memory_usage::heap_size
+    )]
     fn lazy_bound(self, db: &'db dyn Db) -> Option<TypeVarBoundOrConstraints<'db>> {
         let definition = self.definition(db)?;
         let module = parsed_module(db, definition.file(db)).load(db);
@@ -8324,7 +8328,11 @@ impl<'db> TypeVarInstance<'db> {
         Some(TypeVarBoundOrConstraints::UpperBound(ty))
     }
 
-    #[salsa::tracked(heap_size=ruff_memory_usage::heap_size)]
+    #[salsa::tracked(
+        cycle_fn=lazy_bound_or_constraints_cycle_recover,
+        cycle_initial=lazy_bound_or_constraints_cycle_initial,
+        heap_size=ruff_memory_usage::heap_size
+    )]
     fn lazy_constraints(self, db: &'db dyn Db) -> Option<TypeVarBoundOrConstraints<'db>> {
         let definition = self.definition(db)?;
         let module = parsed_module(db, definition.file(db)).load(db);
@@ -8385,7 +8393,7 @@ impl<'db> TypeVarInstance<'db> {
 }
 
 #[allow(clippy::ref_option)]
-fn lazy_bound_cycle_recover<'db>(
+fn lazy_bound_or_constraints_cycle_recover<'db>(
     _db: &'db dyn Db,
     _value: &Option<TypeVarBoundOrConstraints<'db>>,
     _count: u32,
@@ -8394,7 +8402,7 @@ fn lazy_bound_cycle_recover<'db>(
     salsa::CycleRecoveryAction::Iterate
 }
 
-fn lazy_bound_cycle_initial<'db>(
+fn lazy_bound_or_constraints_cycle_initial<'db>(
     _db: &'db dyn Db,
     _self: TypeVarInstance<'db>,
 ) -> Option<TypeVarBoundOrConstraints<'db>> {