Fix crash with nested attrs class (#12872)

Fixes #12868.

mypy/semanal.py

@@ -4505,7 +4505,9 @@ class C:
         """
         # TODO: Forward reference to name imported in class body is not
         #       caught.
-        assert self.statement  # we are at class scope
+        if self.statement is None:
+            # Assume it's fine -- don't have enough context to check
+            return True
         return (node is None
                 or self.is_textually_before_statement(node)
                 or not self.is_defined_in_current_module(node.fullname)

test-data/unit/check-attr.test

@@ -1734,3 +1734,18 @@ class C:
         # E: Unsupported converter, only named functions and types are currently supported
     )
 [builtins fixtures/dict.pyi]
+
+[case testAttrsNestedClass]
+from typing import List
+import attr
+
+@attr.s
+class C:
+    @attr.s
+    class D:
+        pass
+    x = attr.ib(type=List[D])
+
+c = C(x=[C.D()])
+reveal_type(c.x)  # N: Revealed type is "builtins.list[__main__.C.D]"
+[builtins fixtures/list.pyi]