From 9363eeca264b03ffbf4b54aee0ae2fee6b0e70be Mon Sep 17 00:00:00 2001
From: Carl Meyer <carl@astral.sh>
Date: Thu, 28 Aug 2025 09:58:01 -0700
Subject: [PATCH] [ty] add support for cyclic legacy generic protocols (#20125)

## Summary

Just add the necessary Salsa cycle handling.

## Test Plan

Added mdtest.
---
 .../resources/mdtest/protocols.md             | 22 +++++++++++++++++++
 crates/ty_python_semantic/src/types.rs        | 20 ++++++++++++++++-
 2 files changed, 41 insertions(+), 1 deletion(-)

diff --git a/crates/ty_python_semantic/resources/mdtest/protocols.md b/crates/ty_python_semantic/resources/mdtest/protocols.md
index d0b72df047..427868e855 100644
--- a/crates/ty_python_semantic/resources/mdtest/protocols.md
+++ b/crates/ty_python_semantic/resources/mdtest/protocols.md
@@ -2339,6 +2339,28 @@ class Bar(Protocol[S]):
 z: S | Bar[S]
 ```
 
+### Recursive legacy generic protocol
+
+```py
+from typing import Generic, TypeVar, Protocol
+
+T = TypeVar("T")
+
+class P(Protocol[T]):
+    attr: "P[T] | T"
+
+class A(Generic[T]):
+    attr: T
+
+class B(A[P[int]]):
+    pass
+
+def f(b: B):
+    reveal_type(b)  # revealed: B
+    reveal_type(b.attr)  # revealed: P[int]
+    reveal_type(b.attr.attr)  # revealed: P[int] | int
+```
+
 ### Recursive generic protocols with property members
 
 An early version of <https://github.com/astral-sh/ruff/pull/19936> caused stack overflows on this
diff --git a/crates/ty_python_semantic/src/types.rs b/crates/ty_python_semantic/src/types.rs
index 771160099a..beaf5e1ea5 100644
--- a/crates/ty_python_semantic/src/types.rs
+++ b/crates/ty_python_semantic/src/types.rs
@@ -5964,7 +5964,7 @@ impl<'db> Type<'db> {
     /// Note that this does not specialize generic classes, functions, or type aliases! That is a
     /// different operation that is performed explicitly (via a subscript operation), or implicitly
     /// via a call to the generic object.
-    #[salsa::tracked(heap_size=ruff_memory_usage::heap_size)]
+    #[salsa::tracked(heap_size=ruff_memory_usage::heap_size, cycle_fn=apply_specialization_cycle_recover, cycle_initial=apply_specialization_cycle_initial)]
     pub(crate) fn apply_specialization(
         self,
         db: &'db dyn Db,
@@ -6610,6 +6610,24 @@ impl<'db> VarianceInferable<'db> for Type<'db> {
     }
 }
 
+fn apply_specialization_cycle_recover<'db>(
+    _db: &'db dyn Db,
+    _value: &Type<'db>,
+    _count: u32,
+    _self: Type<'db>,
+    _specialization: Specialization<'db>,
+) -> salsa::CycleRecoveryAction<Type<'db>> {
+    salsa::CycleRecoveryAction::Iterate
+}
+
+fn apply_specialization_cycle_initial<'db>(
+    _db: &'db dyn Db,
+    _self: Type<'db>,
+    _specialization: Specialization<'db>,
+) -> Type<'db> {
+    Type::Never
+}
+
 /// A mapping that can be applied to a type, producing another type. This is applied inductively to
 /// the components of complex types.
 ///