Fail gracefully on invalid and/or unsupported recursive type aliases

mypy/semanal.py

@@ -275,6 +275,7 @@
     UnboundType,
     get_proper_type,
     get_proper_types,
+    invalid_recursive_alias,
     is_named_instance,
 )
 from mypy.typevars import fill_typevars
@@ -3075,7 +3076,7 @@ def check_and_set_up_type_alias(self, s: AssignmentStmt) -> bool:
             )
             if not res:
                 return False
-            if self.options.enable_recursive_aliases:
+            if self.options.enable_recursive_aliases and not self.is_func_scope():
                 # Only marking incomplete for top-level placeholders makes recursive aliases like
                 # `A = Sequence[str | A]` valid here, similar to how we treat base classes in class
                 # definitions, allowing `class str(Sequence[str]): ...`
@@ -3119,6 +3120,8 @@ def check_and_set_up_type_alias(self, s: AssignmentStmt) -> bool:
             no_args=no_args,
             eager=eager,
         )
+        if invalid_recursive_alias({alias_node}, alias_node.target):
+            self.fail("Invalid recursive alias: a union item of itself", rvalue)
         if isinstance(s.rvalue, (IndexExpr, CallExpr)):  # CallExpr is for `void = type(None)`
             s.rvalue.analyzed = TypeAliasExpr(alias_node)
             s.rvalue.analyzed.line = s.line
@@ -5552,6 +5555,8 @@ def process_placeholder(self, name: str, kind: str, ctx: Context) -> None:
 
     def cannot_resolve_name(self, name: str, kind: str, ctx: Context) -> None:
         self.fail(f'Cannot resolve {kind} "{name}" (possible cyclic definition)', ctx)
+        if self.options.enable_recursive_aliases and self.is_func_scope():
+            self.note("Recursive types are not allowed at function scope", ctx)
 
     def qualified_name(self, name: str) -> str:
         if self.type is not None:

mypy/semanal_typeargs.py

@@ -30,6 +30,7 @@
     UnpackType,
     get_proper_type,
     get_proper_types,
+    invalid_recursive_alias,
 )
 
 
@@ -68,10 +69,16 @@ def visit_type_alias_type(self, t: TypeAliasType) -> None:
         super().visit_type_alias_type(t)
         if t in self.seen_aliases:
             # Avoid infinite recursion on recursive type aliases.
-            # Note: it is fine to skip the aliases we have already seen in non-recursive types,
-            # since errors there have already already reported.
+            # Note: it is fine to skip the aliases we have already seen in non-recursive
+            # types, since errors there have already been reported.
             return
         self.seen_aliases.add(t)
+        assert t.alias is not None, f"Unfixed type alias {t.type_ref}"
+        if invalid_recursive_alias({t.alias}, t.alias.target):
+            # Fix type arguments for invalid aliases (error is already reported).
+            t.args = []
+            t.alias.target = AnyType(TypeOfAny.from_error)
+            return
         get_proper_type(t).accept(self)
 
     def visit_instance(self, t: Instance) -> None:

mypy/typeanal.py

@@ -82,6 +82,7 @@
     UninhabitedType,
     UnionType,
     UnpackType,
+    bad_type_type_item,
     callable_with_ellipsis,
     get_proper_type,
     union_items,
@@ -374,7 +375,6 @@ def visit_unbound_type_nonoptional(self, t: UnboundType, defining_literal: bool)
                         unexpanded_type=t,
                     )
                 if node.eager:
-                    # TODO: Generate error if recursive (once we have recursive types)
                     res = get_proper_type(res)
                 return res
             elif isinstance(node, TypeInfo):
@@ -487,7 +487,10 @@ def try_analyze_special_unbound_type(self, t: UnboundType, fullname: str) -> Opt
                 type_str = "Type[...]" if fullname == "typing.Type" else "type[...]"
                 self.fail(type_str + " must have exactly one type argument", t)
             item = self.anal_type(t.args[0])
-            return TypeType.make_normalized(item, line=t.line)
+            if bad_type_type_item(item):
+                self.fail("Type[...] can't contain another Type[...]", t)
+                item = AnyType(TypeOfAny.from_error)
+            return TypeType.make_normalized(item, line=t.line, column=t.column)
         elif fullname == "typing.ClassVar":
             if self.nesting_level > 0:
                 self.fail("Invalid type: ClassVar nested inside other type", t)

mypy/types.py

@@ -236,8 +236,6 @@ def is_singleton_type(self) -> bool:
 class TypeAliasType(Type):
     """A type alias to another type.
 
-    NOTE: this is not being used yet, and the implementation is still incomplete.
-
     To support recursive type aliases we don't immediately expand a type alias
     during semantic analysis, but create an instance of this type that records the target alias
     definition node (mypy.nodes.TypeAlias) and type arguments (for generic aliases).
@@ -3197,6 +3195,40 @@ def union_items(typ: Type) -> List[ProperType]:
         return [typ]
 
 
+def invalid_recursive_alias(seen_nodes: Set[mypy.nodes.TypeAlias], target: Type) -> bool:
+    """Flag aliases like A = Union[int, A] (and similar mutual aliases).
+
+    Such aliases don't make much sense, and cause problems in later phases.
+    """
+    if isinstance(target, TypeAliasType):
+        if target.alias in seen_nodes:
+            return True
+        assert target.alias, f"Unfixed type alias {target.type_ref}"
+        return invalid_recursive_alias(seen_nodes | {target.alias}, get_proper_type(target))
+    assert isinstance(target, ProperType)
+    if not isinstance(target, UnionType):
+        return False
+    return any(invalid_recursive_alias(seen_nodes, item) for item in target.items)
+
+
+def bad_type_type_item(item: Type) -> bool:
+    """Prohibit types like Type[Type[...]].
+
+    Such types are explicitly prohibited by PEP 484. Also they cause problems
+    with recursive types like T = Type[T], because internal representation of
+    TypeType item is normalized (i.e. always a proper type).
+    """
+    item = get_proper_type(item)
+    if isinstance(item, TypeType):
+        return True
+    if isinstance(item, UnionType):
+        return any(
+            isinstance(get_proper_type(i), TypeType)
+            for i in flatten_nested_unions(item.items, handle_type_alias_type=True)
+        )
+    return False
+
+
 def is_union_with_any(tp: Type) -> bool:
     """Is this a union with Any or a plain Any type?"""
     tp = get_proper_type(tp)

test-data/unit/check-recursive-types.test

@@ -388,3 +388,33 @@ reveal_type(bar(la))  # N: Revealed type is "__main__.A"
 reveal_type(bar(lla))  # N: Revealed type is "__main__.A"
 reveal_type(bar(llla))  # N: Revealed type is "__main__.A"
 [builtins fixtures/isinstancelist.pyi]
+
+[case testRecursiveAliasesProhibitBadAliases]
+# flags: --enable-recursive-aliases
+from typing import Union, Type, List, TypeVar
+
+NR = List[int]
+NR2 = Union[NR, NR]
+NR3 = Union[NR, Union[NR2, NR2]]
+
+A = Union[B, int]  # E: Invalid recursive alias: a union item of itself
+B = Union[int, A]  # E: Invalid recursive alias: a union item of itself
+def f() -> A: ...
+reveal_type(f())  # N: Revealed type is "Union[Any, builtins.int]"
+
+T = TypeVar("T")
+G = Union[T, G[T]]  # E: Invalid recursive alias: a union item of itself
+def g() -> G[int]: ...
+reveal_type(g())  # N: Revealed type is "Any"
+
+def local() -> None:
+    L = List[Union[int, L]]  # E: Cannot resolve name "L" (possible cyclic definition) \
+                             # N: Recursive types are not allowed at function scope
+    x: L
+    reveal_type(x)  # N: Revealed type is "builtins.list[Union[builtins.int, Any]]"
+
+S = Type[S]  # E: Type[...] cannot contain another Type[...]
+U = Type[Union[int, U]]  # E: Type[...] cannot contain another Type[...]
+x: U
+reveal_type(x)  # N: Revealed type is "Type[Any]"
+[builtins fixtures/isinstancelist.pyi]