[ty] Respect `kw_only` from parent class (#21820)

## Summary

Closes https://github.com/astral-sh/ty/issues/1769.

---------

Co-authored-by: Carl Meyer <carl@astral.sh>

crates/ty_python_semantic/resources/mdtest/dataclasses/dataclasses.md

@@ -701,6 +701,111 @@ Employee("Alice", e_id=1)
 Employee("Alice", 1)  # error: [too-many-positional-arguments]
 ```
 
+### Inherited fields with class-level `kw_only`
+
+When a child dataclass uses `@dataclass(kw_only=True)`, the `kw_only` setting should only apply to
+fields defined in the child class, not to inherited fields from parent classes.
+
+This is a regression test for <https://github.com/astral-sh/ty/issues/1769>.
+
+```toml
+[environment]
+python-version = "3.10"
+```
+
+```py
+from dataclasses import dataclass
+
+@dataclass
+class Inner:
+    inner: int
+
+@dataclass(kw_only=True)
+class Outer(Inner):
+    outer: int
+
+# Inherited field `inner` is positional, new field `outer` is keyword-only
+reveal_type(Outer.__init__)  # revealed: (self: Outer, inner: int, *, outer: int) -> None
+
+Outer(0, outer=5)  # OK
+Outer(inner=0, outer=5)  # Also OK
+# error: [missing-argument]
+# error: [too-many-positional-arguments]
+Outer(0, 5)
+```
+
+This also works when the parent class uses the `KW_ONLY` sentinel:
+
+```py
+from dataclasses import dataclass, KW_ONLY
+
+@dataclass
+class Parent:
+    a: int
+    _: KW_ONLY
+    b: str
+
+@dataclass(kw_only=True)
+class Child(Parent):
+    c: bytes
+
+# `a` is positional (from parent), `b` is keyword-only (from parent's KW_ONLY),
+# `c` is keyword-only (from child's kw_only=True)
+reveal_type(Child.__init__)  # revealed: (self: Child, a: int, *, b: str, c: bytes) -> None
+
+Child(1, b="hello", c=b"world")  # OK
+# error: [missing-argument] "No arguments provided for required parameters `b`, `c`"
+# error: [too-many-positional-arguments]
+Child(1, "hello", b"world")
+```
+
+And when the child class uses the `KW_ONLY` sentinel while inheriting from a parent:
+
+```py
+from dataclasses import dataclass, KW_ONLY
+
+@dataclass
+class Base:
+    x: int
+
+@dataclass
+class Derived(Base):
+    y: str
+    _: KW_ONLY
+    z: bytes
+
+# `x` and `y` are positional, `z` is keyword-only (from Derived's KW_ONLY)
+reveal_type(Derived.__init__)  # revealed: (self: Derived, x: int, y: str, *, z: bytes) -> None
+
+Derived(1, "hello", z=b"world")  # OK
+# error: [missing-argument]
+# error: [too-many-positional-arguments]
+Derived(1, "hello", b"world")
+```
+
+The reverse case also works: when a parent has `kw_only=True` but the child doesn't, the parent's
+fields stay keyword-only while the child's fields are positional:
+
+```py
+from dataclasses import dataclass
+
+@dataclass(kw_only=True)
+class KwOnlyParent:
+    parent_field: int
+
+@dataclass
+class PositionalChild(KwOnlyParent):
+    child_field: str
+
+# `child_field` is positional (child's default), `parent_field` stays keyword-only
+reveal_type(PositionalChild.__init__)  # revealed: (self: PositionalChild, child_field: str, *, parent_field: int) -> None
+
+PositionalChild("hello", parent_field=1)  # OK
+# error: [missing-argument]
+# error: [too-many-positional-arguments]
+PositionalChild("hello", 1)
+```
+
 ### `slots`
 
 If a dataclass is defined with `slots=True`, the `__slots__` attribute is generated as a tuple. It

crates/ty_python_semantic/src/types/class.rs

@@ -21,7 +21,9 @@ use crate::types::constraints::{ConstraintSet, IteratorConstraintsExtension};
 use crate::types::context::InferContext;
 use crate::types::diagnostic::{INVALID_TYPE_ALIAS_TYPE, SUPER_CALL_IN_NAMED_TUPLE_METHOD};
 use crate::types::enums::enum_metadata;
-use crate::types::function::{DataclassTransformerParams, KnownFunction};
+use crate::types::function::{
+    DataclassTransformerFlags, DataclassTransformerParams, KnownFunction,
+};
 use crate::types::generics::{
     GenericContext, InferableTypeVars, Specialization, walk_generic_context, walk_specialization,
 };
@@ -2347,6 +2349,8 @@ impl<'db> ClassLiteral<'db> {
             };
 
         // Dataclass transformer flags can be overwritten using class arguments.
+        // TODO this should be done more generally, not just in `own_synthesized_member`, so that
+        // `dataclass_params` always reflects the transformer params.
         if let Some(transformer_params) = transformer_params.as_mut() {
             if let Some(class_def) = self.definition(db).kind(db).as_class() {
                 let module = parsed_module(db, self.file(db)).load(db);
@@ -2376,6 +2380,8 @@ impl<'db> ClassLiteral<'db> {
 
         let has_dataclass_param = |param| {
             dataclass_params.is_some_and(|params| params.flags(db).contains(param))
+                // TODO if we were correctly initializing `dataclass_params` from the
+                // transformer params, this fallback shouldn't be needed here.
                 || transformer_params.is_some_and(|params| params.flags(db).contains(param))
         };
 
@@ -2455,8 +2461,7 @@ impl<'db> ClassLiteral<'db> {
                     }
                 }
 
-                let is_kw_only = name == "__replace__"
-                    || kw_only.unwrap_or(has_dataclass_param(DataclassFlags::KW_ONLY));
+                let is_kw_only = name == "__replace__" || kw_only.unwrap_or(false);
 
                 // Use the alias name if provided, otherwise use the field name
                 let parameter_name =
@@ -3175,6 +3180,27 @@ impl<'db> ClassLiteral<'db> {
                     }
                 }
 
+                // Resolve the kw_only to the class-level default. This ensures that when fields
+                // are inherited by child classes, they use their defining class's kw_only default.
+                if let FieldKind::Dataclass {
+                    kw_only: ref mut kw @ None,
+                    ..
+                } = field.kind
+                {
+                    let class_kw_only_default = self
+                        .dataclass_params(db)
+                        .is_some_and(|params| params.flags(db).contains(DataclassFlags::KW_ONLY))
+                        // TODO this next part should not be necessary, if we were properly
+                        // initializing `dataclass_params` from the dataclass-transform params, for
+                        // metaclass and base-class-based dataclass-transformers.
+                        || matches!(
+                            field_policy,
+                            CodeGeneratorKind::DataclassLike(Some(transformer_params))
+                                if transformer_params.flags(db).contains(DataclassTransformerFlags::KW_ONLY_DEFAULT)
+                        );
+                    *kw = Some(class_kw_only_default);
+                }
+
                 attributes.insert(symbol.name().clone(), field);
             }
         }