Allow recursive aliases at class scope (#13754)

As `mypy_primer` in #13516 shows,
some people actually use this, and I don't see any good reason to not
allow this. (Note: I tried to enable the same for recursive NamedTuples
and TypedDicts, but this affects how nested classes work in general, so
people will need to use qualified names there).

mypy/semanal.py

@@ -5086,7 +5086,9 @@ class C:
                 X = X  # Initializer refers to outer scope
 
         Nested classes are an exception, since we want to support
-        arbitrary forward references in type annotations.
+        arbitrary forward references in type annotations. Also, we
+        allow forward references to type aliases to support recursive
+        types.
         """
         # TODO: Forward reference to name imported in class body is not
         #       caught.
@@ -5097,7 +5099,7 @@ class C:
             node is None
             or self.is_textually_before_statement(node)
             or not self.is_defined_in_current_module(node.fullname)
-            or isinstance(node, TypeInfo)
+            or isinstance(node, (TypeInfo, TypeAlias))
             or (isinstance(node, PlaceholderNode) and node.becomes_typeinfo)
         )
 

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

@@ -810,3 +810,19 @@ std: STD[str]
 reveal_type(std)  # N: Revealed type is "TypedDict('__main__.STD', {'val': builtins.str, 'other': ..., 'sval': builtins.str, 'one': TypedDict('__main__.TD', {'val': builtins.str, 'other': ...})})"
 [builtins fixtures/dict.pyi]
 [typing fixtures/typing-typeddict.pyi]
+
+[case testRecursiveClassLevelAlias]
+# flags: --enable-recursive-aliases
+from typing import Union, Sequence
+
+class A:
+    Children = Union[Sequence['Children'], 'A', None]
+x: A.Children
+reveal_type(x)  # N: Revealed type is "Union[typing.Sequence[...], __main__.A, None]"
+
+class B:
+    Foo = Sequence[Bar]
+    Bar = Sequence[Foo]
+y: B.Foo
+reveal_type(y)  # N: Revealed type is "typing.Sequence[typing.Sequence[...]]"
+[builtins fixtures/tuple.pyi]