[ty] apply KW_ONLY sentinel only to local fields (#19986)

fix https://github.com/astral-sh/ty/issues/1047

## Summary

This PR fixes how `KW_ONLY` is applied in dataclasses. Previously, the
sentinel leaked into subclasses and incorrectly marked their fields as
keyword-only; now it only affects fields declared in the same class.

```py
from dataclasses import dataclass, KW_ONLY

@dataclass
class D:
    x: int
    _: KW_ONLY
    y: str

@dataclass
class E(D):
    z: bytes

# This should work: x=1 (positional), z=b"foo" (positional), y="foo" (keyword-only)
E(1, b"foo", y="foo")

reveal_type(E.__init__)  # revealed: (self: E, x: int, z: bytes, *, y: str) -> None
```

<!-- What's the purpose of the change? What does it do, and why? -->

## Test Plan

<!-- How was it tested? -->
mdtests

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

@@ -988,6 +988,28 @@ class D:  # error: [duplicate-kw-only]
         z: float
 ```
 
+`KW_ONLY` should only affect fields declared after it within the same class, not fields in
+subclasses:
+
+```py
+from dataclasses import dataclass, KW_ONLY
+
+@dataclass
+class D:
+    x: int
+    _: KW_ONLY
+    y: str
+
+@dataclass
+class E(D):
+    z: bytes
+
+# This should work: x=1 (positional), z=b"foo" (positional), y="foo" (keyword-only)
+E(1, b"foo", y="foo")
+
+reveal_type(E.__init__)  # revealed: (self: E, x: int, z: bytes, *, y: str) -> None
+```
+
 ## Other special cases
 
 ### `dataclasses.dataclass`

crates/ty_python_semantic/resources/mdtest/snapshots/dataclasses.md_-_Dataclasses_-_`dataclasses.KW_ONLY…_(dd1b8f2f71487f16).snap

@@ -49,6 +49,22 @@ mdtest path: crates/ty_python_semantic/resources/mdtest/dataclasses/dataclasses.
 35 |         y: str
 36 |         _2: KW_ONLY
 37 |         z: float
+38 | from dataclasses import dataclass, KW_ONLY
+39 | 
+40 | @dataclass
+41 | class D:
+42 |     x: int
+43 |     _: KW_ONLY
+44 |     y: str
+45 | 
+46 | @dataclass
+47 | class E(D):
+48 |     z: bytes
+49 | 
+50 | # This should work: x=1 (positional), z=b"foo" (positional), y="foo" (keyword-only)
+51 | E(1, b"foo", y="foo")
+52 | 
+53 | reveal_type(E.__init__)  # revealed: (self: E, x: int, z: bytes, *, y: str) -> None
 ```
 
 # Diagnostics
@@ -141,3 +157,15 @@ info: `KW_ONLY` fields: `_1`, `_2`
 info: rule `duplicate-kw-only` is enabled by default
 
 ```
+
+```
+info[revealed-type]: Revealed type
+  --> src/mdtest_snippet.py:53:13
+   |
+51 | E(1, b"foo", y="foo")
+52 |
+53 | reveal_type(E.__init__)  # revealed: (self: E, x: int, z: bytes, *, y: str) -> None
+   |             ^^^^^^^^^^ `(self: E, x: int, z: bytes, *, y: str) -> None`
+   |
+
+```

crates/ty_python_semantic/src/types/class.rs

@@ -1179,6 +1179,16 @@ pub(crate) struct Field<'db> {
     pub(crate) kw_only: Option<bool>,
 }
 
+impl<'db> Field<'db> {
+    /// Returns true if this field is a `dataclasses.KW_ONLY` sentinel.
+    /// <https://docs.python.org/3/library/dataclasses.html#dataclasses.KW_ONLY>
+    pub(crate) fn is_kw_only_sentinel(&self, db: &'db dyn Db) -> bool {
+        self.declared_ty
+            .into_nominal_instance()
+            .is_some_and(|instance| instance.class(db).is_known(db, KnownClass::KwOnly))
+    }
+}
+
 /// Representation of a class definition statement in the AST: either a non-generic class, or a
 /// generic class that has not been specialized.
 ///
@@ -1932,10 +1942,9 @@ impl<'db> ClassLiteral<'db> {
             Type::instance(db, self.apply_optional_specialization(db, specialization));
 
         let signature_from_fields = |mut parameters: Vec<_>, return_ty: Option<Type<'db>>| {
-            let mut kw_only_field_seen = false;
             for (
                 field_name,
-                Field {
+                field @ Field {
                     declared_ty: mut field_ty,
                     mut default_ty,
                     init_only: _,
@@ -1949,14 +1958,10 @@ impl<'db> ClassLiteral<'db> {
                     continue;
                 }
 
-                if field_ty
-                    .into_nominal_instance()
-                    .is_some_and(|instance| instance.class(db).is_known(db, KnownClass::KwOnly))
-                {
+                if field.is_kw_only_sentinel(db) {
                     // Attributes annotated with `dataclass.KW_ONLY` are not present in the synthesized
                     // `__init__` method; they are used to indicate that the following parameters are
                     // keyword-only.
-                    kw_only_field_seen = true;
                     continue;
                 }
 
@@ -2006,9 +2011,7 @@ impl<'db> ClassLiteral<'db> {
                 }
 
                 let is_kw_only = name == "__replace__"
-                    || kw_only.unwrap_or(
-                        has_dataclass_param(DataclassParams::KW_ONLY) || kw_only_field_seen,
-                    );
+                    || kw_only.unwrap_or(has_dataclass_param(DataclassParams::KW_ONLY));
 
                 let mut parameter = if is_kw_only {
                     Parameter::keyword_only(field_name)
@@ -2307,6 +2310,7 @@ impl<'db> ClassLiteral<'db> {
         let table = place_table(db, class_body_scope);
 
         let use_def = use_def_map(db, class_body_scope);
+        let mut kw_only_sentinel_field_seen = false;
         for (symbol_id, declarations) in use_def.all_end_of_scope_symbol_declarations() {
             // Here, we exclude all declarations that are not annotated assignments. We need this because
             // things like function definitions and nested classes would otherwise be considered dataclass
@@ -2351,16 +2355,25 @@ impl<'db> ClassLiteral<'db> {
                     kw_only = field.kw_only(db);
                 }
 
-                attributes.insert(
-                    symbol.name().clone(),
-                    Field {
+                let mut field = Field {
                     declared_ty: attr_ty.apply_optional_specialization(db, specialization),
                     default_ty,
                     init_only: attr.is_init_var(),
                     init,
                     kw_only,
-                    },
-                );
+                };
+
+                // Check if this is a KW_ONLY sentinel and mark subsequent fields as keyword-only
+                if field.is_kw_only_sentinel(db) {
+                    kw_only_sentinel_field_seen = true;
+                }
+
+                // If no explicit kw_only setting and we've seen KW_ONLY sentinel, mark as keyword-only
+                if field.kw_only.is_none() && kw_only_sentinel_field_seen {
+                    field.kw_only = Some(true);
+                }
+
+                attributes.insert(symbol.name().clone(), field);
             }
         }
 

crates/ty_python_semantic/src/types/infer.rs

@@ -90,7 +90,7 @@ use crate::semantic_index::{
     ApplicableConstraints, EnclosingSnapshotResult, SemanticIndex, place_table, semantic_index,
 };
 use crate::types::call::{Binding, Bindings, CallArguments, CallError, CallErrorKind};
-use crate::types::class::{CodeGeneratorKind, Field, MetaclassErrorKind};
+use crate::types::class::{CodeGeneratorKind, MetaclassErrorKind};
 use crate::types::diagnostic::{
     self, CALL_NON_CALLABLE, CONFLICTING_DECLARATIONS, CONFLICTING_METACLASS,
     CYCLIC_CLASS_DEFINITION, DIVISION_BY_ZERO, DUPLICATE_KW_ONLY, INCONSISTENT_MRO,
@@ -1405,31 +1405,16 @@ impl<'db, 'ast> TypeInferenceBuilder<'db, 'ast> {
                 CodeGeneratorKind::from_class(self.db(), class)
             {
                 let specialization = None;
-                let mut kw_only_field_names = vec![];
 
-                for (
-                    name,
-                    Field {
-                        declared_ty: field_ty,
-                        ..
-                    },
-                ) in class.fields(self.db(), specialization, field_policy)
-                {
-                    let Some(instance) = field_ty.into_nominal_instance() else {
-                        continue;
-                    };
+                let kw_only_sentinel_fields: Vec<_> = class
+                    .fields(self.db(), specialization, field_policy)
+                    .into_iter()
+                    .filter_map(|(name, field)| {
+                        field.is_kw_only_sentinel(self.db()).then_some(name)
+                    })
+                    .collect();
 
-                    if !instance
-                        .class(self.db())
-                        .is_known(self.db(), KnownClass::KwOnly)
-                    {
-                        continue;
-                    }
-
-                    kw_only_field_names.push(name);
-                }
-
-                if kw_only_field_names.len() > 1 {
+                if kw_only_sentinel_fields.len() > 1 {
                     // TODO: The fields should be displayed in a subdiagnostic.
                     if let Some(builder) = self
                         .context
@@ -1441,7 +1426,7 @@ impl<'db, 'ast> TypeInferenceBuilder<'db, 'ast> {
 
                         diagnostic.info(format_args!(
                             "`KW_ONLY` fields: {}",
-                            kw_only_field_names
+                            kw_only_sentinel_fields
                                 .iter()
                                 .map(|name| format!("`{name}`"))
                                 .join(", ")