Fix crash on bare Final in dataclass

mypy/plugin.py

@@ -407,6 +407,10 @@ def final_iteration(self) -> bool:
     def is_stub_file(self) -> bool:
         raise NotImplementedError
 
+    @abstractmethod
+    def analyze_simple_literal_type(self, rvalue: Expression, is_final: bool) -> Type | None:
+        raise NotImplementedError
+
 
 # A context for querying for configuration data about a module for
 # cache invalidation purposes.

mypy/plugins/dataclasses.py

@@ -368,7 +368,7 @@ def collect_attributes(self) -> list[DataclassAttribute] | None:
 
             if isinstance(node, TypeAlias):
                 ctx.api.fail(
-                    ("Type aliases inside dataclass definitions " "are not supported at runtime"),
+                    ("Type aliases inside dataclass definitions are not supported at runtime"),
                     node,
                 )
                 # Skip processing this node. This doesn't match the runtime behaviour,
@@ -426,6 +426,23 @@ def collect_attributes(self) -> list[DataclassAttribute] | None:
                 is_kw_only = bool(ctx.api.parse_bool(field_kw_only_param))
 
             known_attrs.add(lhs.name)
+
+            if sym.type is None and node.is_final and node.is_inferred:
+                # This is a special case, assignment like x: Final = 42 is classified
+                # annotated above, but mypy strips the `Final` turning it into x = 42.
+                # We do not support inferred types in dataclasses, so we can try inferring
+                # type for simple literals, and otherwise require an explicit type
+                # argument for Final[...].
+                typ = ctx.api.analyze_simple_literal_type(stmt.rvalue, is_final=True)
+                if typ:
+                    node.type = typ
+                else:
+                    ctx.api.fail(
+                        "Need type argument for Final[...] with non-literal default in dataclass",
+                        stmt,
+                    )
+                    node.type = AnyType(TypeOfAny.from_error)
+
             attrs.append(
                 DataclassAttribute(
                     name=lhs.name,

test-data/unit/check-dataclasses.test

@@ -1796,3 +1796,21 @@ t: Two
 reveal_type(t.__match_args__)  # E: "Two" has no attribute "__match_args__" \
                                # N: Revealed type is "Any"
 [builtins fixtures/dataclasses.pyi]
+
+[case testFinalInDataclass]
+from dataclasses import dataclass
+from typing import Final
+
+@dataclass
+class FirstClass:
+    FIRST_CONST: Final = 3  # OK
+
+@dataclass
+class SecondClass:
+    SECOND_CONST: Final = FirstClass.FIRST_CONST  # E: Need type argument for Final[...] with non-literal default in dataclass
+
+reveal_type(FirstClass().FIRST_CONST)  # N: Revealed type is "Literal[3]?"
+FirstClass().FIRST_CONST = 42  # E: Cannot assign to final attribute "FIRST_CONST"
+reveal_type(SecondClass().SECOND_CONST)  # N: Revealed type is "Literal[3]?"
+SecondClass().SECOND_CONST = 42  # E: Cannot assign to final attribute "SECOND_CONST"
+[builtins fixtures/dataclasses.pyi]