Add __replace__ for dataclasses

mypy/plugins/dataclasses.py

@@ -385,6 +385,9 @@ def transform(self) -> bool:
 
         self._add_dataclass_fields_magic_attribute()
         self._add_internal_replace_method(attributes)
+        if self._api.options.python_version >= (3, 13):
+            self._add_dunder_replace(attributes)
+
         if "__post_init__" in info.names:
             self._add_internal_post_init_method(attributes)
 
@@ -395,6 +398,18 @@ def transform(self) -> bool:
 
         return True
 
+    def _add_dunder_replace(self, attributes: list[DataclassAttribute]) -> None:
+        """Add a `__replace__` method to the class, which is used to replace attributes in the `copy` module."""
+        args = [attr.to_argument(self._cls.info, of="replace") for attr in attributes]
+        type_vars = [tv for tv in self._cls.type_vars]
+        add_method_to_class(
+            self._api,
+            self._cls,
+            "__replace__",
+            args=args,
+            return_type=Instance(self._cls.info, type_vars),
+        )
+
     def _add_internal_replace_method(self, attributes: list[DataclassAttribute]) -> None:
         """
         Stashes the signature of 'dataclasses.replace(...)' for this specific dataclass

test-data/unit/check-dataclasses.test

@@ -2489,3 +2489,37 @@ class Base:
 class Child(Base):
     y: int
 [builtins fixtures/dataclasses.pyi]
+
+[case testDunderReplacePresent]
+# flags: --python-version 3.13
+from dataclasses import dataclass
+
+@dataclass
+class Coords:
+    x: int
+    y: int
+
+
+replaced = Coords(2, 4).__replace__(x=2, y=5)
+reveal_type(replaced)  # N: Revealed type is "__main__.Coords"
+
+replaced = Coords(2, 4).__replace__(x=2)
+reveal_type(replaced)  # N: Revealed type is "__main__.Coords"
+
+Coords(2, 4).__replace__(x="asdf")  # E: Argument "x" to "__replace__" of "Coords" has incompatible type "str"; expected "int"
+Coords(2, 4).__replace__(23)          # E: Too many positional arguments for "__replace__" of "Coords"
+Coords(2, 4).__replace__(23, 25)      # E: Too many positional arguments for "__replace__" of "Coords"
+Coords(2, 4).__replace__(x=23, y=25, z=42)  # E: Unexpected keyword argument "z" for "__replace__" of "Coords"
+
+from typing import Generic, TypeVar
+T = TypeVar('T')
+
+@dataclass
+class Gen(Generic[T]):
+    x: T
+
+replaced_2 = Gen(2).__replace__(x=2)
+reveal_type(replaced_2)  # N: Revealed type is "__main__.Gen[builtins.int]"
+Gen(2).__replace__(x="not an int")  # E: Argument "x" to "__replace__" of "Gen" has incompatible type "str"; expected "int"
+
+[builtins fixtures/tuple.pyi]