From cbfecfaf41c62f3f4b88e1e88c0b0f5946814ea3 Mon Sep 17 00:00:00 2001
From: Douglas Creager <dcreager@dcreager.net>
Date: Mon, 15 Dec 2025 10:25:33 -0500
Subject: [PATCH] [ty] Avoid stack overflow when calculating inferable typevars
 (#21971)

When we calculate which typevars are inferable in a generic context, the
result might include more than the typevars bound by the generic
context. The canonical example is a generic method of a generic class:

```py
class C[A]:
    def method[T](self, t: T): ...
```

Here, the inferable typevar set of `method` contains `Self` and `T`, as
you'd expect. (Those are the typevars bound by the method.) But it also
contains `A@C`, since the implicit `Self` typevar is defined as `Self:
C[A]`. That means when we call `method`, we need to mark `A@C` as
inferable, so that we can determine the correct mapping for `A@C` at the
call site.

Fixes https://github.com/astral-sh/ty/issues/1874
---
 .../mdtest/generics/pep695/classes.md         | 23 +++++++++++++++++++
 .../ty_python_semantic/src/types/generics.rs  | 21 ++++++++++++++---
 2 files changed, 41 insertions(+), 3 deletions(-)

diff --git a/crates/ty_python_semantic/resources/mdtest/generics/pep695/classes.md b/crates/ty_python_semantic/resources/mdtest/generics/pep695/classes.md
index 71e05171e8..06680d2168 100644
--- a/crates/ty_python_semantic/resources/mdtest/generics/pep695/classes.md
+++ b/crates/ty_python_semantic/resources/mdtest/generics/pep695/classes.md
@@ -800,6 +800,29 @@ def func(x: D): ...
 func(G())  # error: [invalid-argument-type]
 ```
 
+### Self-referential protocol with different specialization
+
+This is a minimal reproduction for [ty#1874](https://github.com/astral-sh/ty/issues/1874).
+
+```py
+from __future__ import annotations
+from typing import Protocol
+from ty_extensions import generic_context
+
+class A[S, R](Protocol):
+    def get(self, s: S) -> R: ...
+    def set(self, s: S, r: R) -> S: ...
+    def merge[R2](self, other: A[S, R2]) -> A[S, tuple[R, R2]]: ...
+
+class Impl[S, R](A[S, R]):
+    def foo(self, s: S) -> S:
+        return self.set(s, self.get(s))
+
+reveal_type(generic_context(A.get))  # revealed: ty_extensions.GenericContext[Self@get]
+reveal_type(generic_context(A.merge))  # revealed: ty_extensions.GenericContext[Self@merge, R2@merge]
+reveal_type(generic_context(Impl.foo))  # revealed: ty_extensions.GenericContext[Self@foo]
+```
+
 ## 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
diff --git a/crates/ty_python_semantic/src/types/generics.rs b/crates/ty_python_semantic/src/types/generics.rs
index 58ccd28ca0..d4e6c7a446 100644
--- a/crates/ty_python_semantic/src/types/generics.rs
+++ b/crates/ty_python_semantic/src/types/generics.rs
@@ -23,7 +23,7 @@ use crate::types::{
     KnownClass, KnownInstanceType, MaterializationKind, NormalizedVisitor, Signature, Type,
     TypeContext, TypeMapping, TypeRelation, TypeVarBoundOrConstraints, TypeVarIdentity,
     TypeVarInstance, TypeVarKind, TypeVarVariance, UnionType, declaration_type,
-    walk_bound_type_var_type,
+    walk_type_var_bounds,
 };
 use crate::{Db, FxOrderMap, FxOrderSet};
 
@@ -290,6 +290,18 @@ impl<'db> GenericContext<'db> {
         )
     }
 
+    /// Returns the typevars that are inferable in this generic context. This set might include
+    /// more typevars than the ones directly bound by the generic context. For instance, consider a
+    /// method of a generic class:
+    ///
+    /// ```py
+    /// class C[A]:
+    ///     def method[T](self, t: T):
+    /// ```
+    ///
+    /// In this example, `method`'s generic context binds `Self` and `T`, but its inferable set
+    /// also includes `A@C`. This is needed because at each call site, we need to infer the
+    /// specialized class instance type whose method is being invoked.
     pub(crate) fn inferable_typevars(self, db: &'db dyn Db) -> InferableTypeVars<'db, 'db> {
         #[derive(Default)]
         struct CollectTypeVars<'db> {
@@ -299,7 +311,7 @@ impl<'db> GenericContext<'db> {
 
         impl<'db> TypeVisitor<'db> for CollectTypeVars<'db> {
             fn should_visit_lazy_type_attributes(&self) -> bool {
-                true
+                false
             }
 
             fn visit_bound_type_var_type(
@@ -310,7 +322,10 @@ impl<'db> GenericContext<'db> {
                 self.typevars
                     .borrow_mut()
                     .insert(bound_typevar.identity(db));
-                walk_bound_type_var_type(db, bound_typevar, self);
+                let typevar = bound_typevar.typevar(db);
+                if let Some(bound_or_constraints) = typevar.bound_or_constraints(db) {
+                    walk_type_var_bounds(db, bound_or_constraints, self);
+                }
             }
 
             fn visit_type(&self, db: &'db dyn Db, ty: Type<'db>) {