Fix __post_init__() internal error (#16080)

Fixes #16057.

Author: ikonst

mypy/checker.py

@@ -1076,6 +1076,8 @@ def check_func_item(
 
         if name == "__exit__":
             self.check__exit__return_type(defn)
+        # TODO: the following logic should move to the dataclasses plugin
+        #  https://github.com/python/mypy/issues/15515
         if name == "__post_init__":
             if dataclasses_plugin.is_processed_dataclass(defn.info):
                 dataclasses_plugin.check_post_init(self, defn, defn.info)
@@ -2882,7 +2884,8 @@ def check_assignment(
                     typ = self.expr_checker.accept(rvalue)
                     self.check_match_args(inferred, typ, lvalue)
                 if name == "__post_init__":
-                    if dataclasses_plugin.is_processed_dataclass(self.scope.active_class()):
+                    active_class = self.scope.active_class()
+                    if active_class and dataclasses_plugin.is_processed_dataclass(active_class):
                         self.fail(message_registry.DATACLASS_POST_INIT_MUST_BE_A_FUNCTION, rvalue)
 
             # Defer PartialType's super type checking.

mypy/nodes.py

@@ -515,7 +515,6 @@ def __init__(self) -> None:
         # Original, not semantically analyzed type (used for reprocessing)
         self.unanalyzed_type: mypy.types.ProperType | None = None
         # If method, reference to TypeInfo
-        # TODO: Type should be Optional[TypeInfo]
         self.info = FUNC_NO_INFO
         self.is_property = False
         self.is_class = False

mypy/plugins/dataclasses.py

@@ -1070,8 +1070,8 @@ def replace_function_sig_callback(ctx: FunctionSigContext) -> CallableType:
     )
 
 
-def is_processed_dataclass(info: TypeInfo | None) -> bool:
-    return info is not None and "dataclass" in info.metadata
+def is_processed_dataclass(info: TypeInfo) -> bool:
+    return bool(info) and "dataclass" in info.metadata
 
 
 def check_post_init(api: TypeChecker, defn: FuncItem, info: TypeInfo) -> None:

test-data/unit/check-dataclasses.test

@@ -2280,6 +2280,10 @@ reveal_type(a2)  # N: Revealed type is "__main__.A[builtins.int]"
 
 [builtins fixtures/tuple.pyi]
 
+[case testPostInitNotMethod]
+def __post_init__() -> None:
+    pass
+
 [case testPostInitCorrectSignature]
 from typing import Any, Generic, TypeVar, Callable, Self
 from dataclasses import dataclass, InitVar