fix(transformer): preserve accessor type annotation for emitDecoratorMetadata

`@dec accessor x: T` previously emitted `design:type = Object` regardless of
`T`. Lowering rewrites the accessor into a private storage field plus a
synthesized getter/setter pair and transfers the decorators to the getter,
but it dropped the type annotation. The metadata pass then read the synth
getter's empty `return_type` and fell through to `Object` for every typed
accessor field — including primitives the metadata serializer handles
correctly elsewhere.

Route `accessor.type_annotation` onto the synth getter's `return_type` so
the existing getter path serializes it as `design:type`. The synth setter
(no decorators, no metadata) keeps `None`. `Reflect.getMetadata("design:type", ...)`
now returns the correct constructor for `string` / `number` / `boolean` / etc.

Refs #21922 (bug 7).

Co-Authored-By: Claude <[email protected]>

Author: Dunqing

crates/oxc_transformer/src/decorator/legacy/mod.rs

@@ -48,7 +48,9 @@ mod metadata;
 use std::borrow::Cow;
 use std::mem;
 
-use oxc_allocator::{Address, CloneIn, GetAddress, TakeIn, UnstableAddress, Vec as ArenaVec};
+use oxc_allocator::{
+    Address, Box as ArenaBox, CloneIn, GetAddress, TakeIn, UnstableAddress, Vec as ArenaVec,
+};
 use oxc_ast::{NONE, ast::*};
 use oxc_ast_visit::{Visit, VisitMut};
 use oxc_data_structures::stack::NonEmptyStack;
@@ -359,6 +361,10 @@ impl<'a> LegacyDecorator<'a> {
 
             // Transfer decorators to the getter so legacy decorator transform can process them.
             let decorators = std::mem::replace(&mut accessor.decorators, ctx.ast.vec());
+            // Transfer the type annotation to the getter's return type so that
+            // `emitDecoratorMetadata` can derive `design:type` from it. Without this,
+            // the lowered getter is untyped and `design:type` falls through to `Object`.
+            let type_annotation = accessor.type_annotation.take();
 
             // For computed keys, duplicate the key expression so getter and setter
             // each get their own reference:
@@ -426,6 +432,7 @@ impl<'a> LegacyDecorator<'a> {
                 storage_name,
                 object_binding,
                 class_scope_id,
+                type_annotation,
                 ctx,
             ));
 
@@ -439,6 +446,7 @@ impl<'a> LegacyDecorator<'a> {
                 storage_name,
                 object_binding,
                 class_scope_id,
+                None,
                 ctx,
             ));
         }
@@ -467,6 +475,7 @@ impl<'a> LegacyDecorator<'a> {
         storage_name: Str<'a>,
         object_binding: Option<&BoundIdentifier<'a>>,
         class_scope_id: ScopeId,
+        return_type: Option<ArenaBox<'a, TSTypeAnnotation<'a>>>,
         ctx: &mut TraverseCtx<'a>,
     ) -> ClassElement<'a> {
         let is_getter = kind == MethodDefinitionKind::Get;
@@ -539,7 +548,7 @@ impl<'a> LegacyDecorator<'a> {
             (params, stmt)
         };
 
-        create_class_method(
+        let mut element = create_class_method(
             decorators,
             key,
             kind,
@@ -549,7 +558,13 @@ impl<'a> LegacyDecorator<'a> {
             is_static,
             scope_id,
             ctx,
-        )
+        );
+        if let Some(return_type) = return_type
+            && let ClassElement::MethodDefinition(method) = &mut element
+        {
+            method.value.return_type = Some(return_type);
+        }
+        element
     }
 
     /// Helper method to handle a decorated class element (method, property, or accessor).

tasks/transform_conformance/tests/legacy-decorators/test/fixtures/oxc/metadata/accessor/exec.ts

@@ -0,0 +1,18 @@
+import "reflect-metadata";
+
+function dec() {}
+
+class Entity {
+	@dec accessor name: string = "";
+	@dec accessor count: number = 0;
+	@dec accessor flag: boolean = false;
+	@dec accessor untyped = "x";
+	@dec static accessor sName: string = "";
+}
+
+const e = new Entity();
+expect(Reflect.getMetadata("design:type", e, "name")).toBe(String);
+expect(Reflect.getMetadata("design:type", e, "count")).toBe(Number);
+expect(Reflect.getMetadata("design:type", e, "flag")).toBe(Boolean);
+expect(Reflect.getMetadata("design:type", e, "untyped")).toBe(Object);
+expect(Reflect.getMetadata("design:type", Entity, "sName")).toBe(String);

tasks/transform_conformance/tests/legacy-decorators/test/fixtures/oxc/metadata/accessor/input.ts

@@ -0,0 +1,9 @@
+declare function dec(target: any, propertyKey: string): void;
+
+class Entity {
+	@dec accessor name: string = "";
+	@dec accessor count: number = 0;
+	@dec accessor flag: boolean = false;
+	@dec accessor untyped = "x";
+	@dec static accessor sName: string = "";
+}

tasks/transform_conformance/tests/legacy-decorators/test/fixtures/oxc/metadata/accessor/output.js

@@ -0,0 +1,62 @@
+class Entity {
+  #_name_accessor_storage = "";
+  get name() {
+    return this.#_name_accessor_storage;
+  }
+  set name(value) {
+    this.#_name_accessor_storage = value;
+  }
+  #_count_accessor_storage = 0;
+  get count() {
+    return this.#_count_accessor_storage;
+  }
+  set count(value) {
+    this.#_count_accessor_storage = value;
+  }
+  #_flag_accessor_storage = false;
+  get flag() {
+    return this.#_flag_accessor_storage;
+  }
+  set flag(value) {
+    this.#_flag_accessor_storage = value;
+  }
+  #_untyped_accessor_storage = "x";
+  get untyped() {
+    return this.#_untyped_accessor_storage;
+  }
+  set untyped(value) {
+    this.#_untyped_accessor_storage = value;
+  }
+  static #_sName_accessor_storage = "";
+  static get sName() {
+    return Entity.#_sName_accessor_storage;
+  }
+  static set sName(value) {
+    Entity.#_sName_accessor_storage = value;
+  }
+}
+babelHelpers.decorate([
+  dec,
+  babelHelpers.decorateMetadata("design:type", String),
+  babelHelpers.decorateMetadata("design:paramtypes", [])
+], Entity.prototype, "name", null);
+babelHelpers.decorate([
+  dec,
+  babelHelpers.decorateMetadata("design:type", Number),
+  babelHelpers.decorateMetadata("design:paramtypes", [])
+], Entity.prototype, "count", null);
+babelHelpers.decorate([
+  dec,
+  babelHelpers.decorateMetadata("design:type", Boolean),
+  babelHelpers.decorateMetadata("design:paramtypes", [])
+], Entity.prototype, "flag", null);
+babelHelpers.decorate([
+  dec,
+  babelHelpers.decorateMetadata("design:type", Object),
+  babelHelpers.decorateMetadata("design:paramtypes", [])
+], Entity.prototype, "untyped", null);
+babelHelpers.decorate([
+  dec,
+  babelHelpers.decorateMetadata("design:type", String),
+  babelHelpers.decorateMetadata("design:paramtypes", [])
+], Entity, "sName", null);