Report final dataclass fields that are never initialized

mypy/plugins/dataclasses.py

@@ -654,6 +654,18 @@ def collect_attributes(self) -> list[DataclassAttribute] | None:
             elif not isinstance(stmt.rvalue, TempNode):
                 has_default = True
 
+            if (
+                node.is_final
+                and has_field_call
+                and not is_in_init
+                and not has_default
+                and not is_init_var
+                and self._spec is _TRANSFORM_SPEC_FOR_DATACLASSES
+            ):
+                # A final attribute can only be assigned in __init__(), so if it is
+                # excluded from __init__() and has no default value, it can never be set.
+                self._api.fail("Final field must have a default value if init=False", stmt.rvalue)
+
             if not has_default and self._spec is _TRANSFORM_SPEC_FOR_DATACLASSES:
                 # Make all non-default dataclass attributes implicit because they are de-facto
                 # set on self in the generated __init__(), not in the class body. On the other

test-data/unit/check-dataclasses.test

@@ -2952,6 +2952,27 @@ class Bad2:
     x: InitVar[Final[int]]  # E: Final can be only used as an outermost qualifier in a variable annotation
 [builtins fixtures/dict.pyi]
 
+[case testDataclassFinalFieldNotInInit]
+from dataclasses import dataclass, field
+from typing import Final
+
+@dataclass
+class Bad:
+    x: Final[int] = field(init=False)  # E: Final field must have a default value if init=False
+
+@dataclass
+class GoodDefault:
+    x: Final[int] = field(init=False, default=0)
+
+@dataclass
+class GoodFactory:
+    x: Final[int] = field(init=False, default_factory=int)
+
+@dataclass
+class GoodPlain:
+    x: Final[int] = 0
+[builtins fixtures/tuple.pyi]
+
 [case testNoCrashOnNestedSpecialFormsWithAnnotations]
 import dataclasses
 from typing import ClassVar, NamedTuple