Fix dataclass/protocol crash on joining types (#15629)

The root cause is hacky creation of incomplete symbols; instead
switching to `add_method_to_class` which does the necessary
housekeeping.

Fixes #15618.

mypy/checker.py

@@ -1200,9 +1200,10 @@ def check_func_def(
                     elif isinstance(arg_type, TypeVarType):
                         # Refuse covariant parameter type variables
                         # TODO: check recursively for inner type variables
-                        if arg_type.variance == COVARIANT and defn.name not in (
-                            "__init__",
-                            "__new__",
+                        if (
+                            arg_type.variance == COVARIANT
+                            and defn.name not in ("__init__", "__new__", "__post_init__")
+                            and not is_private(defn.name)  # private methods are not inherited
                         ):
                             ctx: Context = arg_type
                             if ctx.line < 0:

mypy/plugins/dataclasses.py

@@ -2,7 +2,7 @@
 
 from __future__ import annotations
 
-from typing import TYPE_CHECKING, Final, Iterator
+from typing import TYPE_CHECKING, Final, Iterator, Literal
 
 from mypy import errorcodes, message_registry
 from mypy.expandtype import expand_type, expand_type_by_instance
@@ -86,7 +86,7 @@
     field_specifiers=("dataclasses.Field", "dataclasses.field"),
 )
 _INTERNAL_REPLACE_SYM_NAME: Final = "__mypy-replace"
-_INTERNAL_POST_INIT_SYM_NAME: Final = "__mypy-__post_init__"
+_INTERNAL_POST_INIT_SYM_NAME: Final = "__mypy-post_init"
 
 
 class DataclassAttribute:
@@ -118,14 +118,33 @@ def __init__(
         self.is_neither_frozen_nor_nonfrozen = is_neither_frozen_nor_nonfrozen
         self._api = api
 
-    def to_argument(self, current_info: TypeInfo) -> Argument:
-        arg_kind = ARG_POS
-        if self.kw_only and self.has_default:
-            arg_kind = ARG_NAMED_OPT
-        elif self.kw_only and not self.has_default:
-            arg_kind = ARG_NAMED
-        elif not self.kw_only and self.has_default:
-            arg_kind = ARG_OPT
+    def to_argument(
+        self, current_info: TypeInfo, *, of: Literal["__init__", "replace", "__post_init__"]
+    ) -> Argument:
+        if of == "__init__":
+            arg_kind = ARG_POS
+            if self.kw_only and self.has_default:
+                arg_kind = ARG_NAMED_OPT
+            elif self.kw_only and not self.has_default:
+                arg_kind = ARG_NAMED
+            elif not self.kw_only and self.has_default:
+                arg_kind = ARG_OPT
+        elif of == "replace":
+            arg_kind = ARG_NAMED if self.is_init_var and not self.has_default else ARG_NAMED_OPT
+        elif of == "__post_init__":
+            # We always use `ARG_POS` without a default value, because it is practical.
+            # Consider this case:
+            #
+            # @dataclass
+            # class My:
+            #     y: dataclasses.InitVar[str] = 'a'
+            #     def __post_init__(self, y: str) -> None: ...
+            #
+            # We would be *required* to specify `y: str = ...` if default is added here.
+            # But, most people won't care about adding default values to `__post_init__`,
+            # because it is not designed to be called directly, and duplicating default values
+            # for the sake of type-checking is unpleasant.
+            arg_kind = ARG_POS
         return Argument(
             variable=self.to_var(current_info),
             type_annotation=self.expand_type(current_info),
@@ -236,7 +255,7 @@ def transform(self) -> bool:
             and attributes
         ):
             args = [
-                attr.to_argument(info)
+                attr.to_argument(info, of="__init__")
                 for attr in attributes
                 if attr.is_in_init and not self._is_kw_only_type(attr.type)
             ]
@@ -375,70 +394,26 @@ def _add_internal_replace_method(self, attributes: list[DataclassAttribute]) ->
         Stashes the signature of 'dataclasses.replace(...)' for this specific dataclass
         to be used later whenever 'dataclasses.replace' is called for this dataclass.
         """
-        arg_types: list[Type] = []
-        arg_kinds = []
-        arg_names: list[str | None] = []
-
-        info = self._cls.info
-        for attr in attributes:
-            attr_type = attr.expand_type(info)
-            assert attr_type is not None
-            arg_types.append(attr_type)
-            arg_kinds.append(
-                ARG_NAMED if attr.is_init_var and not attr.has_default else ARG_NAMED_OPT
-            )
-            arg_names.append(attr.name)
-
-        signature = CallableType(
-            arg_types=arg_types,
-            arg_kinds=arg_kinds,
-            arg_names=arg_names,
-            ret_type=NoneType(),
-            fallback=self._api.named_type("builtins.function"),
-        )
-
-        info.names[_INTERNAL_REPLACE_SYM_NAME] = SymbolTableNode(
-            kind=MDEF, node=FuncDef(typ=signature), plugin_generated=True
+        add_method_to_class(
+            self._api,
+            self._cls,
+            _INTERNAL_REPLACE_SYM_NAME,
+            args=[attr.to_argument(self._cls.info, of="replace") for attr in attributes],
+            return_type=NoneType(),
+            is_staticmethod=True,
         )
 
     def _add_internal_post_init_method(self, attributes: list[DataclassAttribute]) -> None:
-        arg_types: list[Type] = [fill_typevars(self._cls.info)]
-        arg_kinds = [ARG_POS]
-        arg_names: list[str | None] = ["self"]
-
-        info = self._cls.info
-        for attr in attributes:
-            if not attr.is_init_var:
-                continue
-            attr_type = attr.expand_type(info)
-            assert attr_type is not None
-            arg_types.append(attr_type)
-            # We always use `ARG_POS` without a default value, because it is practical.
-            # Consider this case:
-            #
-            # @dataclass
-            # class My:
-            #     y: dataclasses.InitVar[str] = 'a'
-            #     def __post_init__(self, y: str) -> None: ...
-            #
-            # We would be *required* to specify `y: str = ...` if default is added here.
-            # But, most people won't care about adding default values to `__post_init__`,
-            # because it is not designed to be called directly, and duplicating default values
-            # for the sake of type-checking is unpleasant.
-            arg_kinds.append(ARG_POS)
-            arg_names.append(attr.name)
-
-        signature = CallableType(
-            arg_types=arg_types,
-            arg_kinds=arg_kinds,
-            arg_names=arg_names,
-            ret_type=NoneType(),
-            fallback=self._api.named_type("builtins.function"),
-            name="__post_init__",
-        )
-
-        info.names[_INTERNAL_POST_INIT_SYM_NAME] = SymbolTableNode(
-            kind=MDEF, node=FuncDef(typ=signature), plugin_generated=True
+        add_method_to_class(
+            self._api,
+            self._cls,
+            _INTERNAL_POST_INIT_SYM_NAME,
+            args=[
+                attr.to_argument(self._cls.info, of="__post_init__")
+                for attr in attributes
+                if attr.is_init_var
+            ],
+            return_type=NoneType(),
         )
 
     def add_slots(
@@ -1120,20 +1095,18 @@ def is_processed_dataclass(info: TypeInfo | None) -> bool:
 def check_post_init(api: TypeChecker, defn: FuncItem, info: TypeInfo) -> None:
     if defn.type is None:
         return
-
-    ideal_sig = info.get_method(_INTERNAL_POST_INIT_SYM_NAME)
-    if ideal_sig is None or ideal_sig.type is None:
-        return
-
-    # We set it ourself, so it is always fine:
-    assert isinstance(ideal_sig.type, ProperType)
-    assert isinstance(ideal_sig.type, FunctionLike)
-    # Type of `FuncItem` is always `FunctionLike`:
     assert isinstance(defn.type, FunctionLike)
 
+    ideal_sig_method = info.get_method(_INTERNAL_POST_INIT_SYM_NAME)
+    assert ideal_sig_method is not None and ideal_sig_method.type is not None
+    ideal_sig = ideal_sig_method.type
+    assert isinstance(ideal_sig, ProperType)  # we set it ourselves
+    assert isinstance(ideal_sig, CallableType)
+    ideal_sig = ideal_sig.copy_modified(name="__post_init__")
+
     api.check_override(
         override=defn.type,
-        original=ideal_sig.type,
+        original=ideal_sig,
         name="__post_init__",
         name_in_super="__post_init__",
         supertype="dataclass",

test-data/unit/check-dataclasses.test

@@ -744,6 +744,17 @@ s: str = a.bar()  # E: Incompatible types in assignment (expression has type "in
 
 [builtins fixtures/dataclasses.pyi]
 
+[case testDataclassGenericCovariant]
+from dataclasses import dataclass
+from typing import Generic, TypeVar
+
+T_co = TypeVar("T_co", covariant=True)
+
+@dataclass
+class MyDataclass(Generic[T_co]):
+    a: T_co
+
+[builtins fixtures/dataclasses.pyi]
 
 [case testDataclassUntypedGenericInheritance]
 # flags: --python-version 3.7
@@ -2449,3 +2460,15 @@ class Test(Protocol):
     def reset(self) -> None:
         self.x = DEFAULT
 [builtins fixtures/dataclasses.pyi]
+
+[case testProtocolNoCrashOnJoining]
+from dataclasses import dataclass
+from typing import Protocol
+
+@dataclass
+class MyDataclass(Protocol): ...
+
+a: MyDataclass
+b = [a, a]  # trigger joining the types
+
+[builtins fixtures/dataclasses.pyi]

test-data/unit/deps.test

@@ -1388,7 +1388,7 @@ class B(A):
 <m.A.(abstract)> -> <m.B.__init__>, m
 <m.A.__dataclass_fields__> -> <m.B.__dataclass_fields__>
 <m.A.__init__> -> <m.B.__init__>, m.B.__init__
-<m.A.__mypy-replace> -> <m.B.__mypy-replace>
+<m.A.__mypy-replace> -> <m.B.__mypy-replace>, m.B.__mypy-replace
 <m.A.__new__> -> <m.B.__new__>
 <m.A.x> -> <m.B.x>
 <m.A.y> -> <m.B.y>
@@ -1420,7 +1420,7 @@ class B(A):
 <m.A.__dataclass_fields__> -> <m.B.__dataclass_fields__>
 <m.A.__init__> -> <m.B.__init__>, m.B.__init__
 <m.A.__match_args__> -> <m.B.__match_args__>
-<m.A.__mypy-replace> -> <m.B.__mypy-replace>
+<m.A.__mypy-replace> -> <m.B.__mypy-replace>, m.B.__mypy-replace
 <m.A.__new__> -> <m.B.__new__>
 <m.A.x> -> <m.B.x>
 <m.A.y> -> <m.B.y>