python · ilevkivskyi · May 10, 2026 · May 14, 2026 · May 14, 2026 · May 15, 2026
diff --git a/mypy/checker.py b/mypy/checker.py
@@ -8529,6 +8529,9 @@ def lookup_fully_qualified_or_none(self, fullname: str, /) -> SymbolTableNode |
         except KeyError:
             return None
 
+    def record_fixed_type(self, fixed: TypeInfo | TypeAlias) -> None:
+        pass
+
     def fail(
         self,
         msg: str,

diff --git a/mypy/checkexpr.py b/mypy/checkexpr.py
@@ -4915,10 +4915,11 @@ def visit_type_application(self, tapp: TypeApplication) -> Type:
             if tapp.expr.node.python_3_12_type_alias:
                 return self.type_alias_type_type()
             # Subscription of a (generic) alias in runtime context, expand the alias.
-            item = instantiate_type_alias(
+            item, _ = instantiate_type_alias(
                 tapp.expr.node,
                 tapp.types,
                 self.chk.fail,
+                self.chk.note,
                 tapp.expr.node.no_args,
                 tapp,
                 self.chk.options,
@@ -4983,17 +4984,16 @@ class LongName(Generic[T]): ...
         #     A = List[Tuple[T, T]]
         #     x = A() <- same as List[Tuple[Any, Any]], see PEP 484.
         disallow_any = self.chk.options.disallow_any_generics and self.is_callee
-        item = get_proper_type(
-            set_any_tvars(
-                alias,
-                [],
-                ctx.line,
-                ctx.column,
-                self.chk.options,
-                disallow_any=disallow_any,
-                fail=self.msg.fail,
-            )
+        item, _ = set_any_tvars(
+            alias,
+            [],
+            ctx.line,
+            ctx.column,
+            self.chk.options,
+            disallow_any=disallow_any,
+            fail=self.msg.fail,
         )
+        item = get_proper_type(item)
         if isinstance(item, Instance):
             # Normally we get a callable type (or overloaded) with .is_type_obj() true
             # representing the class's constructor

diff --git a/mypy/message_registry.py b/mypy/message_registry.py
@@ -180,6 +180,7 @@ def with_additional_msg(self, info: str) -> ErrorMessage:
 IMPLICIT_GENERIC_ANY_BUILTIN: Final = (
     'Implicit generic "Any". Use "{}" and specify generic parameters'
 )
+NO_CYCLIC_DEFAULT: Final = "Cyclic type variable defaults are not supported"
 INVALID_UNPACK: Final = "{} cannot be unpacked (must be tuple or TypeVarTuple)"
 INVALID_UNPACK_POSITION: Final = "Unpack is only valid in a variadic position"
 INVALID_PARAM_SPEC_LOCATION: Final = "Invalid location for ParamSpec {}"

diff --git a/mypy/nodes.py b/mypy/nodes.py
@@ -3145,7 +3145,15 @@ class TypeVarLikeExpr(SymbolNode, Expression):
     Note that they are constructed by the semantic analyzer.
     """
 
-    __slots__ = ("_name", "_fullname", "upper_bound", "default", "variance", "is_new_style")
+    __slots__ = (
+        "_name",
+        "_fullname",
+        "upper_bound",
+        "default",
+        "variance",
+        "is_new_style",
+        "default_depends",
+    )
 
     _name: str
     _fullname: str
@@ -3160,6 +3168,9 @@ class TypeVarLikeExpr(SymbolNode, Expression):
     # TypeVar(..., contravariant=True) defines a contravariant type
     # variable.
     variance: int
+    # Record instances and type aliases that appear bare/implicit in the default value
+    # of this type variable. This is needed to detect recursive type variable defaults.
+    default_depends: set[TypeInfo | TypeAlias] | None
 
     def __init__(
         self,
@@ -3178,6 +3189,7 @@ def __init__(
         self.default = default
         self.variance = variance
         self.is_new_style = is_new_style
+        self.default_depends = None
 
     @property
     def name(self) -> str:
@@ -3655,6 +3667,7 @@ class is generic then it will be a type constructor of higher kind.
         "is_type_check_only",
         "deprecated",
         "type_object_type",
+        "default_depends",
     )
 
     _fullname: str  # Fully qualified name
@@ -3816,6 +3829,16 @@ class is generic then it will be a type constructor of higher kind.
     # appears in runtime context.
     type_object_type: mypy.types.FunctionLike | None
 
+    # Type variables whose defaults depend on defaults of type variables in other classes
+    # and type aliases. We keep track of this to safely handle situations like this one:
+    #     class C[T = D]: ...
+    #     class D[S = C]: ...
+    #     x: C
+    # Since we apply fix_instance() eagerly, inferring a precise type is quite tricky.
+    # Therefore, we infer the type of `x` as `C[D[Any]]` to avoid infinite recursion.
+    # Keys are type variable full names.
+    default_depends: dict[str, set[TypeAlias | TypeInfo]]
+
     FLAGS: Final = [
         "is_abstract",
         "is_enum",
@@ -3877,6 +3900,7 @@ def __init__(self, names: SymbolTable, defn: ClassDef, module_name: str) -> None
         self.is_type_check_only = False
         self.deprecated = None
         self.type_object_type = None
+        self.default_depends = {}
 
     def add_type_vars(self) -> None:
         self.has_type_var_tuple_type = False
@@ -4542,6 +4566,7 @@ def f(x: B[T]) -> T: ...  # without T, Any would be used here
         "eager",
         "tvar_tuple_index",
         "python_3_12_type_alias",
+        "default_depends",
     )
 
     __match_args__ = ("name", "target", "alias_tvars", "no_args")
@@ -4574,6 +4599,8 @@ def __init__(
         self.eager = eager
         self.python_3_12_type_alias = python_3_12_type_alias
         self.tvar_tuple_index = None
+        # This plays the same role as TypeInfo.default_depends attribute.
+        self.default_depends: dict[str, set[TypeAlias | TypeInfo]] = {}
         for i, t in enumerate(alias_tvars):
             if isinstance(t, mypy.types.TypeVarTupleType):
                 self.tvar_tuple_index = i