fix: allow Final[T] instance fields in dataclasses to depend on type variables

PEP 591 and the dataclass spec say that a Final field declared in a
dataclass body is an *instance attribute*, not a class attribute, unless
explicitly annotated with ClassVar. The checker was unconditionally
raising 'Final name declared in class body cannot depend on type
variables' for any Final[T] in a generic class body, which is correct
for plain classes but wrong for dataclasses.

fix: when the active class has 'dataclass_tag' in its metadata and the
lvalue is a Var that is not marked is_classvar, skip the error.
non-dataclass classes and explicit ClassVar fields are unaffected.

fixes #21637

Author: adhavan18

mypy/checker.py

@@ -3398,9 +3398,21 @@ def visit_assignment_stmt(self, s: AssignmentStmt) -> None:
             s.is_final_def
             and s.type
             and not has_no_typevars(s.type)
-            and self.scope.active_class() is not None
+            and (active_class := self.scope.active_class()) is not None
         ):
-            self.fail(message_registry.DEPENDENT_FINAL_IN_CLASS_BODY, s)
+            # Final[T] instance fields in dataclasses are not class attributes, so
+            # they are allowed to depend on type variables (PEP 591 / dataclass spec).
+            # Only suppress the error when the field is a dataclass instance field
+            # (not explicitly ClassVar).
+            lv = s.lvalues[0] if s.lvalues else None
+            is_dataclass_instance_field = (
+                "dataclass_tag" in active_class.metadata
+                and isinstance(lv, RefExpr)
+                and isinstance(lv.node, Var)
+                and not lv.node.is_classvar
+            )
+            if not is_dataclass_instance_field:
+                self.fail(message_registry.DEPENDENT_FINAL_IN_CLASS_BODY, s)
 
         if s.unanalyzed_type and not self.in_checked_function():
             self.msg.annotation_in_unchecked_function(context=s)

test-data/unit/check-final.test

@@ -1322,3 +1322,35 @@ class S9(S2, S4):  # E: Class "S9" has incompatible disjoint bases
     pass
 
 [builtins fixtures/tuple.pyi]
+
+[case testFinalDefiningTypevarsDataclassInstanceField]
+# Regression test for https://github.com/python/mypy/issues/21637
+# Final[T] instance fields in dataclasses must NOT trigger the
+# "cannot depend on type variables" error, because they are instance
+# attributes, not class attributes.
+from dataclasses import dataclass
+from typing import Final, Generic, TypeVar
+
+T = TypeVar("T")
+
+@dataclass(frozen=True)
+class A(Generic[T]):
+    value: Final[T]  # OK — instance field in a dataclass
+
+@dataclass(frozen=True)
+class B(Generic[T]):
+    value: T  # OK — control case without Final
+[builtins fixtures/dataclasses.pyi]
+[typing fixtures/typing-medium.pyi]
+
+[case testFinalDefiningTypevarsNonDataclassStillErrors]
+# Final[T] in a plain (non-dataclass) generic class body must still raise the error.
+from typing import Any, Final, Generic, TypeVar
+
+T = TypeVar("T")
+d: Any
+
+class C(Generic[T]):
+    x: Final[T] = d  # E: Final name declared in class body cannot depend on type variables
+[builtins fixtures/dataclasses.pyi]
+[typing fixtures/typing-medium.pyi]