Fix cached_property access through classes

mypy/cache.py

@@ -72,7 +72,7 @@
 from mypy_extensions import u8
 
 # High-level cache layout format
-CACHE_VERSION: Final = 11
+CACHE_VERSION: Final = 12
 
 # Type used internally to represent errors:
 #   (path, line, column, end_line, end_column, severity, message, code)

mypy/checkmember.py

@@ -1301,6 +1301,20 @@ def analyze_class_attribute_access(
                 t, mx, cast(Decorator, node.node).var, itype, is_class=is_classmethod
             )
 
+        proper_t = get_proper_type(t)
+        if (
+            is_decorated
+            and cast(Decorator, node.node).is_cached_property
+            and isinstance(proper_t, CallableType)
+        ):
+            # ``cached_property.__get__(None, owner)`` returns the descriptor
+            # itself, unlike a regular property access. Preserve that type so
+            # class-level attributes such as ``A.value.attrname`` are valid.
+            cached_property = Instance(
+                mx.chk.lookup_typeinfo("functools.cached_property"), [proper_t.ret_type]
+            )
+            return apply_class_attr_hook(mx, hook, cached_property)
+
         result = t
         # __set__ is not called on class objects.
         if not mx.is_lvalue:

mypy/nodes.py

@@ -1314,7 +1314,14 @@ class Decorator(SymbolNode, Statement):
     A single Decorator object can include any number of function decorators.
     """
 
-    __slots__ = ("func", "decorators", "original_decorators", "var", "is_overload")
+    __slots__ = (
+        "func",
+        "decorators",
+        "original_decorators",
+        "var",
+        "is_overload",
+        "is_cached_property",
+    )
 
     __match_args__ = ("decorators", "var", "func")
 
@@ -1333,6 +1340,7 @@ def __init__(self, func: FuncDef, decorators: list[Expression], var: Var) -> Non
         self.original_decorators = decorators.copy()
         self.var = var
         self.is_overload = False
+        self.is_cached_property = False
 
     @property
     def name(self) -> str:
@@ -1363,20 +1371,23 @@ def serialize(self) -> JsonDict:
             "func": self.func.serialize(),
             "var": self.var.serialize(),
             "is_overload": self.is_overload,
+            "is_cached_property": self.is_cached_property,
         }
 
     @classmethod
     def deserialize(cls, data: JsonDict) -> Decorator:
         assert data[".class"] == "Decorator"
         dec = Decorator(FuncDef.deserialize(data["func"]), [], Var.deserialize(data["var"]))
         dec.is_overload = data["is_overload"]
+        dec.is_cached_property = data.get("is_cached_property", False)
         return dec
 
     def write(self, data: WriteBuffer) -> None:
         write_tag(data, DECORATOR)
         self.func.write(data)
         self.var.write(data)
         write_bool(data, self.is_overload)
+        write_bool(data, self.is_cached_property)
         write_tag(data, END_TAG)
 
     @classmethod
@@ -1387,6 +1398,7 @@ def read(cls, data: ReadBuffer) -> Decorator:
         var = Var.read(data)
         dec = Decorator(func, [], var)
         dec.is_overload = read_bool(data)
+        dec.is_cached_property = read_bool(data)
         assert read_tag(data) == END_TAG
         return dec
 

mypy/semanal.py

@@ -1779,6 +1779,7 @@ def visit_decorator(self, dec: Decorator) -> None:
                     dec.func.abstract_status = IS_ABSTRACT
                 elif refers_to_fullname(d, "functools.cached_property"):
                     dec.var.is_settable_property = True
+                    dec.is_cached_property = True
                 self.check_decorated_function_is_method("property", dec)
             elif refers_to_fullname(d, "typing.no_type_check"):
                 dec.var.type = AnyType(TypeOfAny.special_form)

test-data/unit/check-functools.test

@@ -125,6 +125,30 @@ _T = TypeVar('_T')
 class cached_property(Generic[_T]): ...
 [builtins fixtures/property.pyi]
 
+[case testCachedPropertyClassAccess]
+from functools import cached_property
+
+class A:
+    @cached_property
+    def value(self) -> int: ...
+
+    @classmethod
+    def name(cls) -> str:
+        reveal_type(cls.value)  # N: Revealed type is "functools.cached_property[builtins.int]"
+        return cls.value.attrname or ""
+
+reveal_type(A.value)  # N: Revealed type is "functools.cached_property[builtins.int]"
+[file functools.pyi]
+from typing import Any, Generic, TypeVar, overload
+_T = TypeVar('_T')
+class cached_property(Generic[_T]):
+    attrname: str | None
+    @overload
+    def __get__(self, instance: None, owner: type[Any] | None = ...) -> cached_property[_T]: ...
+    @overload
+    def __get__(self, instance: object, owner: type[Any] | None = ...) -> _T: ...
+[builtins fixtures/property.pyi]
+
 [case testTotalOrderingWithForwardReference]
 from typing import Generic, Any, TypeVar
 import functools