[ty] fix infinite recursion with generic type aliases (#20969)

Co-authored-by: Alex Waygood <Alex.Waygood@Gmail.com>

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

@@ -170,3 +170,64 @@ type X[T: X] = T
 def _(x: X):
     assert x
 ```
+
+## Recursive generic type aliases
+
+```py
+type RecursiveList[T] = T | list[RecursiveList[T]]
+
+r1: RecursiveList[int] = 1
+r2: RecursiveList[int] = [1, [1, 2, 3]]
+# error: [invalid-assignment] "Object of type `Literal["a"]` is not assignable to `RecursiveList[int]`"
+r3: RecursiveList[int] = "a"
+# error: [invalid-assignment]
+r4: RecursiveList[int] = ["a"]
+# TODO: this should be an error
+r5: RecursiveList[int] = [1, ["a"]]
+
+def _(x: RecursiveList[int]):
+    if isinstance(x, list):
+        # TODO: should be `list[RecursiveList[int]]
+        reveal_type(x[0])  # revealed: int | list[Any]
+    if isinstance(x, list) and isinstance(x[0], list):
+        # TODO: should be `list[RecursiveList[int]]`
+        reveal_type(x[0])  # revealed: list[Any]
+```
+
+Assignment checks respect structural subtyping, i.e. type aliases with the same structure are
+assignable to each other.
+
+```py
+# This is structurally equivalent to RecursiveList[T].
+type RecursiveList2[T] = T | list[T | list[RecursiveList[T]]]
+# This is not structurally equivalent to RecursiveList[T].
+type RecursiveList3[T] = T | list[list[RecursiveList[T]]]
+
+def _(x: RecursiveList[int], y: RecursiveList2[int]):
+    r1: RecursiveList2[int] = x
+    # error: [invalid-assignment]
+    r2: RecursiveList3[int] = x
+
+    r3: RecursiveList[int] = y
+    # error: [invalid-assignment]
+    r4: RecursiveList3[int] = y
+```
+
+It is also possible to handle divergent type aliases that are not actually have instances.
+
+```py
+# The type variable `T` has no meaning here, it's just to make sure it works correctly.
+type DivergentList[T] = list[DivergentList[T]]
+
+d1: DivergentList[int] = []
+# error: [invalid-assignment]
+d2: DivergentList[int] = [1]
+# error: [invalid-assignment]
+d3: DivergentList[int] = ["a"]
+# TODO: this should be an error
+d4: DivergentList[int] = [[1]]
+
+def _(x: DivergentList[int]):
+    d1: DivergentList[int] = [x]
+    d2: DivergentList[int] = x[0]
+```

crates/ty_python_semantic/src/types.rs

@@ -6772,7 +6772,11 @@ impl<'db> Type<'db> {
             Type::TypeIs(type_is) => type_is.with_type(db, type_is.return_type(db).apply_type_mapping(db, type_mapping, tcx)),
 
             Type::TypeAlias(alias) => {
-                visitor.visit(self, || alias.value_type(db).apply_type_mapping_impl(db, type_mapping, tcx, visitor))
+                // Do not call `value_type` here. `value_type` does the specialization internally, so `apply_type_mapping` is performed without `visitor` inheritance.
+                // In the case of recursive type aliases, this leads to infinite recursion.
+                // Instead, call `raw_value_type` and perform the specialization after the `visitor` cache has been created.
+                let value_type = visitor.visit(self, || alias.raw_value_type(db).apply_type_mapping_impl(db, type_mapping, tcx, visitor));
+                alias.apply_function_specialization(db, value_type).apply_type_mapping_impl(db, type_mapping, tcx, visitor)
             }
 
             Type::ModuleLiteral(_)
@@ -10716,31 +10720,12 @@ impl<'db> PEP695TypeAliasType<'db> {
     }
 
     /// The RHS type of a PEP-695 style type alias with specialization applied.
-    #[salsa::tracked(cycle_initial=value_type_cycle_initial, heap_size=ruff_memory_usage::heap_size)]
     pub(crate) fn value_type(self, db: &'db dyn Db) -> Type<'db> {
-        let value_type = self.raw_value_type(db);
+        self.apply_function_specialization(db, self.raw_value_type(db))
-
-        if let Some(generic_context) = self.generic_context(db) {
-            let specialization = self
-                .specialization(db)
-                .unwrap_or_else(|| generic_context.default_specialization(db, None));
-
-            value_type.apply_specialization(db, specialization)
-        } else {
-            value_type
-        }
     }
 
     /// The RHS type of a PEP-695 style type alias with *no* specialization applied.
-    ///
+    #[salsa::tracked(cycle_initial=value_type_cycle_initial, heap_size=ruff_memory_usage::heap_size)]
-    /// ## Warning
-    ///
-    /// This uses the semantic index to find the definition of the type alias. This means that if the
-    /// calling query is not in the same file as this type alias is defined in, then this will create
-    /// a cross-module dependency directly on the full AST which will lead to cache
-    /// over-invalidation.
-    /// This method also calls the type inference functions, and since type aliases can have recursive structures,
-    /// we should be careful not to create infinite recursions in this method (or make it tracked if necessary).
     pub(crate) fn raw_value_type(self, db: &'db dyn Db) -> Type<'db> {
         let scope = self.rhs_scope(db);
         let module = parsed_module(db, scope.file(db)).load(db);
@@ -10750,6 +10735,17 @@ impl<'db> PEP695TypeAliasType<'db> {
         definition_expression_type(db, definition, &type_alias_stmt_node.node(&module).value)
     }
 
+    fn apply_function_specialization(self, db: &'db dyn Db, ty: Type<'db>) -> Type<'db> {
+        if let Some(generic_context) = self.generic_context(db) {
+            let specialization = self
+                .specialization(db)
+                .unwrap_or_else(|| generic_context.default_specialization(db, None));
+            ty.apply_specialization(db, specialization)
+        } else {
+            ty
+        }
+    }
+
     pub(crate) fn apply_specialization(
         self,
         db: &'db dyn Db,
@@ -10939,6 +10935,13 @@ impl<'db> TypeAliasType<'db> {
         }
     }
 
+    fn apply_function_specialization(self, db: &'db dyn Db, ty: Type<'db>) -> Type<'db> {
+        match self {
+            TypeAliasType::PEP695(type_alias) => type_alias.apply_function_specialization(db, ty),
+            TypeAliasType::ManualPEP695(_) => ty,
+        }
+    }
+
     pub(crate) fn apply_specialization(
         self,
         db: &'db dyn Db,
@@ -11799,6 +11802,9 @@ type CovariantAlias[T] = Covariant[T]
 type ContravariantAlias[T] = Contravariant[T]
 type InvariantAlias[T] = Invariant[T]
 type BivariantAlias[T] = Bivariant[T]
+
+type RecursiveAlias[T] = None | list[RecursiveAlias[T]]
+type RecursiveAlias2[T] = None | list[T] | list[RecursiveAlias2[T]]
 "#,
         )
         .unwrap();
@@ -11829,5 +11835,19 @@ type BivariantAlias[T] = Bivariant[T]
                 .variance_of(&db, get_bound_typevar(&db, bivariant)),
             TypeVarVariance::Bivariant
         );
+
+        let recursive = get_type_alias(&db, "RecursiveAlias");
+        assert_eq!(
+            KnownInstanceType::TypeAliasType(TypeAliasType::PEP695(recursive))
+                .variance_of(&db, get_bound_typevar(&db, recursive)),
+            TypeVarVariance::Bivariant
+        );
+
+        let recursive2 = get_type_alias(&db, "RecursiveAlias2");
+        assert_eq!(
+            KnownInstanceType::TypeAliasType(TypeAliasType::PEP695(recursive2))
+                .variance_of(&db, get_bound_typevar(&db, recursive2)),
+            TypeVarVariance::Invariant
+        );
     }
 }