Move dataclass kw_only fields to the end of the signature (#19018)

Fixes #19017. Fixes #17731.

This is a rather naive change: python does that at runtime. `kw_only`
args can be in any order, and non-kwonly args should remain sorted as-is
(stable sort). I don't understand why this was only done in presence of
a parent dataclass - AFAIC kwonly fields work that way since `kw_only`
was introduced in py3.10.

The test I changed was invalid and asserted a false positive to the best
of my knowledge.

Author: sterliakov

mypy/plugins/dataclasses.py

@@ -546,7 +546,6 @@ def collect_attributes(self) -> list[DataclassAttribute] | None:
         # in the parent. We can implement this via a dict without disrupting the attr order
         # because dicts preserve insertion order in Python 3.7+.
         found_attrs: dict[str, DataclassAttribute] = {}
-        found_dataclass_supertype = False
         for info in reversed(cls.info.mro[1:-1]):
             if "dataclass_tag" in info.metadata and "dataclass" not in info.metadata:
                 # We haven't processed the base class yet. Need another pass.
@@ -556,7 +555,6 @@ def collect_attributes(self) -> list[DataclassAttribute] | None:
 
             # Each class depends on the set of attributes in its dataclass ancestors.
             self._api.add_plugin_dependency(make_wildcard_trigger(info.fullname))
-            found_dataclass_supertype = True
 
             for data in info.metadata["dataclass"]["attributes"]:
                 name: str = data["name"]
@@ -720,8 +718,7 @@ def collect_attributes(self) -> list[DataclassAttribute] | None:
             )
 
         all_attrs = list(found_attrs.values())
-        if found_dataclass_supertype:
-            all_attrs.sort(key=lambda a: a.kw_only)
+        all_attrs.sort(key=lambda a: a.kw_only)
 
         # Third, ensure that arguments without a default don't follow
         # arguments that have a default and that the KW_ONLY sentinel

test-data/unit/check-dataclass-transform.test

@@ -265,7 +265,7 @@ class Foo:
 
 Foo(a=5, b_=1)  # E: Unexpected keyword argument "a" for "Foo"
 Foo(a_=1, b_=1, noinit=1)  # E: Unexpected keyword argument "noinit" for "Foo"
-Foo(1, 2, 3)  # E: Too many positional arguments for "Foo"
+Foo(1, 2, 3) # (a, b, unused1)
 foo = Foo(1, 2, kwonly=3)
 reveal_type(foo.noinit)  # N: Revealed type is "builtins.int"
 reveal_type(foo.unused1)  # N: Revealed type is "builtins.int"

test-data/unit/check-dataclasses.test

@@ -460,14 +460,16 @@ from dataclasses import dataclass, field, KW_ONLY
 class Application:
     _: KW_ONLY
     name: str = 'Unnamed'
-    rating: int = field(kw_only=False)  # E: Attributes without a default cannot follow attributes with one
+    rating: int = field(kw_only=False)
 
 Application(name='name', rating=5)
-Application()  # E: Missing positional argument "name" in call to "Application"
-Application('name')  # E: Too many positional arguments for "Application" # E: Too few arguments for "Application"
-Application('name', 123)  # E: Too many positional arguments for "Application"
-Application('name', rating=123)  # E: Too many positional arguments for "Application"
-
+Application()  # E: Missing positional argument "rating" in call to "Application"
+Application(123)
+Application('name')  # E: Argument 1 to "Application" has incompatible type "str"; expected "int"
+Application('name', 123)  # E: Too many positional arguments for "Application" \
+                          # E: Argument 1 to "Application" has incompatible type "str"; expected "int" \
+                          # E: Argument 2 to "Application" has incompatible type "int"; expected "str"
+Application(123, rating=123)  # E: "Application" gets multiple values for keyword argument "rating"
 [builtins fixtures/dataclasses.pyi]
 
 [case testDataclassesOrderingKwOnlyWithSentinelAndSubclass]
@@ -2618,3 +2620,30 @@ raw_target: object
 if isinstance(raw_target, type) and dataclasses.is_dataclass(raw_target):
     reveal_type(raw_target)  # N: Revealed type is "type[dataclasses.DataclassInstance]"
 [builtins fixtures/tuple.pyi]
+
+[case testDataclassKwOnlyArgsLast]
+from dataclasses import dataclass, field
+
+@dataclass
+class User:
+    id: int = field(kw_only=True)
+    name: str
+
+User("Foo", id=0)
+[builtins fixtures/tuple.pyi]
+
+[case testDataclassKwOnlyArgsDefaultAllowedNonLast]
+from dataclasses import dataclass, field
+
+@dataclass
+class User:
+    id: int = field(kw_only=True, default=0)
+    name: str
+
+User()  # E: Missing positional argument "name" in call to "User"
+User("")
+User(0)  # E: Argument 1 to "User" has incompatible type "int"; expected "str"
+User("", 0)  # E: Too many positional arguments for "User"
+User("", id=0)
+User("", name="")  # E: "User" gets multiple values for keyword argument "name"
+[builtins fixtures/tuple.pyi]