From 0280949000ee448f17a1c2b5d802d9ad569301ac Mon Sep 17 00:00:00 2001
From: Alex Waygood <Alex.Waygood@Gmail.com>
Date: Wed, 3 Dec 2025 19:01:42 +0000
Subject: [PATCH] [ty] fix panic when attempting to infer the variance of a
 PEP-695 class that depends on a recursive type aliases and also somehow
 protocols (#21778)

Fixes https://github.com/astral-sh/ty/issues/1716.

## Test plan

I added a corpus snippet that causes us to panic on `main` (I tested by
running `cargo run -p ty_python_semantic --test=corpus` without the fix
applied).
---
 .../resources/corpus/cyclic_pep695_variance.py     | 14 ++++++++++++++
 crates/ty_python_semantic/src/types/class.rs       | 11 ++++++++++-
 2 files changed, 24 insertions(+), 1 deletion(-)
 create mode 100644 crates/ty_python_semantic/resources/corpus/cyclic_pep695_variance.py

diff --git a/crates/ty_python_semantic/resources/corpus/cyclic_pep695_variance.py b/crates/ty_python_semantic/resources/corpus/cyclic_pep695_variance.py
new file mode 100644
index 0000000000..70f354337d
--- /dev/null
+++ b/crates/ty_python_semantic/resources/corpus/cyclic_pep695_variance.py
@@ -0,0 +1,14 @@
+from typing import Protocol
+
+class A(Protocol):
+    @property
+    def f(self): ...
+
+type Recursive = int | tuple[Recursive, ...]
+
+class B[T: A]: ...
+
+class C[T: A](A):
+    x: tuple[Recursive, ...]
+
+class D(B[C]): ...
diff --git a/crates/ty_python_semantic/src/types/class.rs b/crates/ty_python_semantic/src/types/class.rs
index 0abb33c54f..5ebf92fb06 100644
--- a/crates/ty_python_semantic/src/types/class.rs
+++ b/crates/ty_python_semantic/src/types/class.rs
@@ -340,9 +340,18 @@ impl<'db> From<GenericAlias<'db>> for Type<'db> {
     }
 }
 
+fn variance_of_cycle_initial<'db>(
+    _db: &'db dyn Db,
+    _id: salsa::Id,
+    _self: GenericAlias<'db>,
+    _typevar: BoundTypeVarInstance<'db>,
+) -> TypeVarVariance {
+    TypeVarVariance::Bivariant
+}
+
 #[salsa::tracked]
 impl<'db> VarianceInferable<'db> for GenericAlias<'db> {
-    #[salsa::tracked(heap_size=ruff_memory_usage::heap_size)]
+    #[salsa::tracked(heap_size=ruff_memory_usage::heap_size, cycle_initial=variance_of_cycle_initial)]
     fn variance_of(self, db: &'db dyn Db, typevar: BoundTypeVarInstance<'db>) -> TypeVarVariance {
         let origin = self.origin(db);