Fix typing problem in ExpressionStateMixin

1 files changed, 8 insertions, 3 deletions

diff --git a/polymatrix/expressionstate/mixins.py b/polymatrix/expressionstate/mixins.py
index ffaea0a..99b2cb1 100644
--- a/polymatrix/expressionstate/mixins.py
+++ b/polymatrix/expressionstate/mixins.py
@@ -2,7 +2,7 @@ from __future__ import annotations
 from typing import TYPE_CHECKING
 
 from abc import abstractmethod
-from typing import NamedTuple
+from typing import NamedTuple, cast
 from math import prod
 import dataclasses
 
@@ -42,8 +42,13 @@ class ExpressionStateMixin(
         size = prod(var.shape)
         index = IndexRange(start=self.n_variables, end=self.n_variables + size)
 
-        return dataclasses.replace( # type: ignore[type-var]
-            self,
+        # FIXME: this is the only way the typechecker is happy, and it does
+        # make sense indeed because ExpressionState is not a dataclass
+        # The typechecker should be somehow informed that this is an ABC that
+        # will eventually become a dataclass, but of course it can't know that
+        from .impl import ExpressionStateImpl
+        return dataclasses.replace(
+            cast(ExpressionStateImpl, self),
             n_variables=self.n_variables + size,
             indices={**self.indices, var: index}
         ), index