[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