Add check for dataclass field default ordering

conformance/third_party/conformance.exp

@@ -3286,6 +3286,16 @@
       "stop_column": 12,
       "stop_line": 53
     },
+    {
+      "code": -2,
+      "column": 5,
+      "concise_description": "Dataclass field 'c' without a default may not follow dataclass field with a default",
+      "description": "Dataclass field 'c' without a default may not follow dataclass field with a default",
+      "line": 58,
+      "name": "bad-class-definition",
+      "stop_column": 6,
+      "stop_line": 58
+    },
     {
       "code": -2,
       "column": 4,
@@ -4004,6 +4014,26 @@
       "stop_column": 37,
       "stop_line": 52
     },
+    {
+      "code": -2,
+      "column": 5,
+      "concise_description": "Dataclass field 'b' without a default may not follow dataclass field with a default",
+      "description": "Dataclass field 'b' without a default may not follow dataclass field with a default",
+      "line": 61,
+      "name": "bad-class-definition",
+      "stop_column": 6,
+      "stop_line": 61
+    },
+    {
+      "code": -2,
+      "column": 5,
+      "concise_description": "Dataclass field 'b' without a default may not follow dataclass field with a default",
+      "description": "Dataclass field 'b' without a default may not follow dataclass field with a default",
+      "line": 67,
+      "name": "bad-class-definition",
+      "stop_column": 6,
+      "stop_line": 67
+    },
     {
       "code": -2,
       "column": 13,

conformance/third_party/conformance.result

@@ -145,6 +145,7 @@
   "dataclasses_kwonly.py": [
     "Line 32: Unexpected errors ['Missing argument `a` in function `DC2.__init__`', 'Expected 0 positional arguments, got 1 in function `DC2.__init__`']",
     "Line 35: Unexpected errors ['Missing argument `a` in function `DC2.__init__`', 'Expected 0 positional arguments, got 1 in function `DC2.__init__`']",
+    "Line 58: Unexpected errors [\"Dataclass field 'c' without a default may not follow dataclass field with a default\"]",
     "Line 61: Unexpected errors ['Missing argument `c` in function `DC4.__init__`', 'Argument `float` is not assignable to parameter `b` with type `int` in function `DC4.__init__`', 'Multiple values for argument `b` in function `DC4.__init__`']"
   ],
   "dataclasses_order.py": [],
@@ -199,8 +200,6 @@
     "Line 79: Unexpected errors ['`<` is not supported between `Customer2` and `Customer2`\\n  Cannot find `__lt__` or `__gt__`']"
   ],
   "dataclasses_usage.py": [
-    "Lines 58, 61: Expected error (tag 'DC1')",
-    "Lines 64, 67: Expected error (tag 'DC2')",
     "Lines 70, 73: Expected error (tag 'DC3')"
   ],
   "directives_assert_type.py": [],

conformance/third_party/results.json

@@ -3,7 +3,7 @@
   "pass": 60,
   "fail": 76,
   "pass_rate": 0.44,
-  "differences": 356,
+  "differences": 355,
   "passing": [
     "aliases_explicit.py",
     "aliases_newtype.py",
@@ -82,15 +82,15 @@
     "constructors_callable.py": 12,
     "dataclasses_descriptors.py": 1,
     "dataclasses_final.py": 3,
-    "dataclasses_kwonly.py": 3,
+    "dataclasses_kwonly.py": 4,
     "dataclasses_postinit.py": 2,
     "dataclasses_slots.py": 5,
     "dataclasses_transform_class.py": 7,
     "dataclasses_transform_converter.py": 7,
     "dataclasses_transform_field.py": 2,
     "dataclasses_transform_func.py": 6,
     "dataclasses_transform_meta.py": 7,
-    "dataclasses_usage.py": 3,
+    "dataclasses_usage.py": 1,
     "directives_deprecated.py": 10,
     "enums_behaviors.py": 4,
     "enums_members.py": 4,

pyrefly/lib/alt/class/dataclass.rs

@@ -19,6 +19,8 @@ use crate::alt::class::class_field::DataclassMember;
 use crate::alt::types::class_metadata::ClassMetadata;
 use crate::alt::types::class_metadata::ClassSynthesizedField;
 use crate::alt::types::class_metadata::ClassSynthesizedFields;
+use crate::error;
+use crate::error::collector::ErrorCollector;
 use crate::types::callable::BoolKeywords;
 use crate::types::callable::Callable;
 use crate::types::callable::DataclassKeywords;
@@ -55,7 +57,11 @@ impl<'a, Ans: LookupAnswer> AnswersSolver<'a, Ans> {
         all_fields
     }
 
-    pub fn get_dataclass_synthesized_fields(&self, cls: &Class) -> Option<ClassSynthesizedFields> {
+    pub fn get_dataclass_synthesized_fields(
+        &self,
+        cls: &Class,
+        errors: &ErrorCollector,
+    ) -> Option<ClassSynthesizedFields> {
         let metadata = self.get_metadata_for_class(cls);
         let dataclass = metadata.dataclass_metadata()?;
         let mut fields = SmallMap::new();
@@ -66,6 +72,7 @@ impl<'a, Ans: LookupAnswer> AnswersSolver<'a, Ans> {
                     cls,
                     &dataclass.fields,
                     dataclass.kws.is_set(&DataclassKeywords::KW_ONLY),
+                    errors,
                 ),
             );
         }
@@ -137,17 +144,39 @@ impl<'a, Ans: LookupAnswer> AnswersSolver<'a, Ans> {
         cls: &Class,
         fields: &SmallSet<Name>,
         kw_only: bool,
+        errors: &ErrorCollector,
     ) -> ClassSynthesizedField {
         let mut params = vec![self.class_self_param(cls, false)];
+        let mut has_seen_default = false;
+
         for (name, field, field_flags) in self.iter_fields(cls, fields, true) {
             if field_flags.is_set(&DataclassKeywords::INIT) {
-                params.push(field.as_param(
-                    &name,
-                    field_flags.is_set(&DataclassKeywords::DEFAULT),
-                    field_flags.is_set(&(DataclassKeywords::KW_ONLY.0, kw_only)),
-                ));
+                let has_default = field_flags.is_set(&DataclassKeywords::DEFAULT);
+                let is_kw_only = field_flags.is_set(&(DataclassKeywords::KW_ONLY.0, kw_only));
+
+                if !is_kw_only {
+                    if !has_default && has_seen_default {
+                        if let Some(range) = cls.field_decl_range(&name) {
+                            self.error(
+                                errors,
+                                range,
+                                error::kind::ErrorKind::BadClassDefinition,
+                                None,
+                                format!(
+                                    "Dataclass field '{}' without a default may not follow dataclass field with a default", name
+                                ),
+                            );
+                        }
+                    }
+                    if has_default {
+                        has_seen_default = true;
+                    }
+                }
+
+                params.push(field.as_param(&name, has_default, is_kw_only));
             }
         }
+
         let ty = Type::Function(Box::new(Function {
             signature: Callable::list(ParamList::new(params), Type::None),
             metadata: FuncMetadata::def(

pyrefly/lib/alt/solve.rs

@@ -1363,7 +1363,7 @@ impl<'a, Ans: LookupAnswer> AnswersSolver<'a, Ans> {
                 if let Some(new_fields) = self.get_typed_dict_synthesized_fields(cls) {
                     fields = fields.combine(new_fields);
                 }
-                if let Some(new_fields) = self.get_dataclass_synthesized_fields(cls) {
+                if let Some(new_fields) = self.get_dataclass_synthesized_fields(cls, errors) {
                     fields = fields.combine(new_fields);
                 }
                 if let Some(new_fields) = self.get_named_tuple_synthesized_fields(cls) {