Self type of ClassVar lost through TypeVar

[asottile-sentry]

this is very similar to #16826 but appears to be slightly different

Bug Report

Self appears to be decaying to the TypeVar bound instead of the TypeVar itself. my specific example looks similar to django models construction but is isolated from django-stubs itself to make a minimal reproduction

it works great for the subclass (as demonstrated by the first reveal_type) -- just not for the TypeVar

To Reproduce

from typing import TypeVar, Generic, ClassVar, Self

M = TypeVar("M", bound="B", covariant=True)

class Related(Generic[M]):
    def get(self) -> M:
        raise NotImplementedError

class B:
    objects: ClassVar[Related[Self]] = Related()

class C(B):
    pass

reveal_type(C.objects)

def of_m(cls: type[M]) -> M:
    return cls.objects.get()

Expected Behavior

(no errors)

Actual Behavior

t.py:15: note: Revealed type is "t.Related[t.C]"
t.py:18: error: Incompatible return value type (got "B", expected "M")  [return-value]
Found 1 error in 1 file (checked 1 source file)

Your Environment

• Mypy version used: 1.10.0
• Mypy command-line flags: (none)
• Mypy configuration options from mypy.ini (and other config files): (n/a)
• Python version used: 3.12.2

[asottile-sentry]

this seems to make this particular example work -- and passes the existing tests

diff --git a/mypy/checkmember.py b/mypy/checkmember.py
index 7525db2..2ed9af1 100644
--- a/mypy/checkmember.py
+++ b/mypy/checkmember.py
@@ -1064,7 +1064,13 @@ def analyze_class_attribute_access(
             # In the above example this means that we infer following types:
             #     C.x -> Any
             #     C[int].x -> int
-            t = get_proper_type(expand_self_type(node.node, t, itype))
+            if isinstance(mx.self_type, CallableType):
+                self_type = mx.self_type.ret_type
+            elif isinstance(mx.self_type, TypeType):
+                self_type = mx.self_type.item
+            else:
+                self_type = itype
+            t = get_proper_type(expand_self_type(node.node, t, self_type))
             t = erase_typevars(expand_type_by_instance(t, isuper), {tv.id for tv in def_vars})
 
         is_classmethod = (is_decorated and cast(Decorator, node.node).func.is_class) or (

[asottile]

via ba5c279