fix(transformer/legacy-decorator): preserve accessor type annotation for emitDecoratorMetadata (#21966)

Fixes bug 7 of https://github.com/oxc-project/oxc/issues/21922.

## The bug

`@dec accessor x: T` always emitted `design:type = Object` regardless of `T`:

```ts
class Entity {
  @dec accessor name: string = "";   // tsc: design:type = String
  @dec accessor count: number = 0;   // tsc: design:type = Number
}
```

`Reflect.getMetadata("design:type", entity, "name")` returned `Object` instead of `String`. Frameworks reading `design:type` (NestJS Swagger, class-validator, TypeORM column metadata) silently degraded for every typed accessor field.

## Cause

`lower_accessor_properties` (in `enter_class`) rewrites
```ts
@dec accessor x: T = init;
```
into three members — a private storage field, a synthesized getter, and a synthesized setter — and transfers the decorators to the getter so the existing decorator pipeline picks them up. But it dropped the type annotation: the synth getter was constructed with `return_type: NONE`.

The metadata pass runs later and takes the getter branch:

```rust
if method.kind.is_get() {
    (self.serialize_type_annotation(method.value.return_type.as_ref(), ctx), None)
}
```

`return_type` is `None` because lowering discarded it, so `serialize_type_annotation` correctly fell through to `Object` for what it sees as an untyped getter.

The metadata pass's `enter_accessor_property` dispatch exists, but it never fires because the `AccessorProperty` no longer exists by the time traversal reaches the class body.

## Fix

Stop dropping the type annotation. In `lower_accessor_properties`:

```rust
let decorators = std::mem::replace(&mut accessor.decorators, ctx.ast.vec());
let type_annotation = accessor.type_annotation.take();
```

Thread `type_annotation` through `create_accessor_method` to the getter (setter passes `None`). To avoid a post-construction patch on the synthesized `MethodDefinition`, `create_class_method` now takes a `return_type: Option<ArenaBox<TSTypeAnnotation>>` parameter and threads it directly into the function's return-type slot. The other caller (`create_class_constructor_with_params`) passes `None`.

The lowered AST is now self-consistent — the synth getter for `accessor x: T` reads as a getter that returns `T`, which is what it semantically is, and the existing getter-metadata path serializes it correctly.

## Out of scope (architectural limit)

`type Uuid = string; @dec accessor x: Uuid` still emits `Object`, same as for non-accessor properties. Resolving type-alias references to their primitive constituent requires a name-based alias-resolution pass (the same one that `tsc`'s checker provides via `getTypeReferenceSerializationKind`). That's bug 1 of #21922 and a separate change from this fix; flagged here so reviewers don't mistake it for an omission.

## Test plan

- [x] Conformance fixture `oxc/metadata/accessor` covers primitives, untyped (correctly stays `Object`), `string[]` → `Array`, `static accessor`, and computed-key (`@dec accessor ["computed"]: number`)
- [x] `cargo test -p oxc_transformer` — 31 + 5 passed
- [x] `cargo run -p oxc_transform_conformance -- --filter legacy-decorators` — 73 pass / 23 fail (baseline 72/24, +1 new fixture, **0 regressions**)
- [x] `cargo run -p oxc_transform_conformance -- --exec` — accessor exec test verifies `Reflect.getMetadata` returns the correct constructor at runtime
- [x] `cargo fmt -p oxc_transformer`

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;
@@ -544,6 +553,7 @@ impl<'a> LegacyDecorator<'a> {
             key,
             kind,
             params,
+            return_type,
             ctx.ast.vec1(body_stmt),
             computed,
             is_static,

crates/oxc_transformer/src/utils/ast_builder.rs

@@ -203,6 +203,7 @@ pub fn create_class_constructor_with_params<'a>(
         ),
         MethodDefinitionKind::Constructor,
         params,
+        None,
         stmts,
         false,
         false,
@@ -217,6 +218,7 @@ pub fn create_class_method<'a>(
     key: PropertyKey<'a>,
     kind: MethodDefinitionKind,
     params: ArenaBox<'a, FormalParameters<'a>>,
+    return_type: Option<ArenaBox<'a, TSTypeAnnotation<'a>>>,
     stmts: ArenaVec<'a, Statement<'a>>,
     computed: bool,
     is_static: bool,
@@ -238,7 +240,7 @@ pub fn create_class_method<'a>(
             NONE,
             NONE,
             params,
-            NONE,
+            return_type,
             Some(ctx.ast.alloc_function_body(SPAN, ctx.ast.vec(), stmts)),
             scope_id,
         ),

tasks/coverage/snapshots/semantic_misc.snap

@@ -18,6 +18,9 @@ rebuilt        : ScopeId(5): Some(ScopeId(0))
 Scope parent mismatch:
 after transform: ScopeId(4): Some(ScopeId(1))
 rebuilt        : ScopeId(7): Some(ScopeId(0))
+Unresolved reference IDs mismatch for "String":
+after transform: [ReferenceId(11), ReferenceId(12)]
+rebuilt        : [ReferenceId(18)]
 
 semantic Error: tasks/coverage/misc/pass/oxc-1288.ts
 Bindings mismatch:

tasks/coverage/snapshots/semantic_typescript.snap

@@ -7150,7 +7150,7 @@ Bindings mismatch:
 after transform: ScopeId(3): []
 rebuilt        : ScopeId(3): ["_class"]
 Symbol reference IDs mismatch for "_decorateMetadata":
-after transform: SymbolId(16): [ReferenceId(12), ReferenceId(13), ReferenceId(15), ReferenceId(16), ReferenceId(24), ReferenceId(25)]
+after transform: SymbolId(16): [ReferenceId(12), ReferenceId(13), ReferenceId(14), ReferenceId(15), ReferenceId(22), ReferenceId(23)]
 rebuilt        : SymbolId(2): []
 Symbol reference IDs mismatch for "dec":
 after transform: SymbolId(0): [ReferenceId(1), ReferenceId(2), ReferenceId(3), ReferenceId(4)]
@@ -7159,7 +7159,7 @@ Symbol scope ID mismatch for "_class":
 after transform: SymbolId(8): ScopeId(0)
 rebuilt        : SymbolId(14): ScopeId(3)
 Unresolved references mismatch:
-after transform: ["DecoratorContext", "Object", "require"]
+after transform: ["DecoratorContext", "require"]
 rebuilt        : ["require"]
 
 semantic Error: tasks/coverage/typescript/tests/cases/compiler/esNextWeakRefs_IterableWeakMap.ts

tasks/transform_conformance/snapshots/oxc.snap.md

@@ -1,6 +1,6 @@
 commit: 6402dbbf
 
-Passed: 228/376
+Passed: 229/377
 
 # All Passed:
 * babel-plugin-transform-class-static-block
@@ -647,7 +647,7 @@ x Output mismatch
 x Output mismatch
 
 
-# legacy-decorators (8/95)
+# legacy-decorators (9/96)
 * oxc/accessor/input.ts
 x Output mismatch
 

tasks/transform_conformance/snapshots/oxc_exec.snap.md

@@ -2,7 +2,7 @@ commit: 6402dbbf
 
 node: v24.14.0
 
-Passed: 14 of 16 (87.50%)
+Passed: 15 of 17 (88.24%)
 
 Failures:
 

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

@@ -0,0 +1,22 @@
+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 accessor list: string[] = [];
+	@dec static accessor sName: string = "";
+	@dec accessor ["computed"]: number = 0;
+}
+
+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", e, "list")).toBe(Array);
+expect(Reflect.getMetadata("design:type", Entity, "sName")).toBe(String);
+expect(Reflect.getMetadata("design:type", e, "computed")).toBe(Number);

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

@@ -0,0 +1,11 @@
+function dec() {}
+
+class Entity {
+	@dec accessor name: string = "";
+	@dec accessor count: number = 0;
+	@dec accessor flag: boolean = false;
+	@dec accessor untyped = "x";
+	@dec accessor list: string[] = [];
+	@dec static accessor sName: string = "";
+	@dec accessor ["computed"]: number = 0;
+}

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

@@ -0,0 +1,87 @@
+function dec() {}
+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;
+  }
+  #_list_accessor_storage = [];
+  get list() {
+    return this.#_list_accessor_storage;
+  }
+  set list(value) {
+    this.#_list_accessor_storage = value;
+  }
+  static #_sName_accessor_storage = "";
+  static get sName() {
+    return Entity.#_sName_accessor_storage;
+  }
+  static set sName(value) {
+    Entity.#_sName_accessor_storage = value;
+  }
+  #_computed_computed_accessor_storage = 0;
+  get ["computed"]() {
+    return this.#_computed_computed_accessor_storage;
+  }
+  set ["computed"](value) {
+    this.#_computed_computed_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", Array),
+  babelHelpers.decorateMetadata("design:paramtypes", [])
+], Entity.prototype, "list", null);
+babelHelpers.decorate([
+  dec,
+  babelHelpers.decorateMetadata("design:type", String),
+  babelHelpers.decorateMetadata("design:paramtypes", [])
+], Entity, "sName", null);
+babelHelpers.decorate([
+  dec,
+  babelHelpers.decorateMetadata("design:type", Number),
+  babelHelpers.decorateMetadata("design:paramtypes", [])
+], Entity.prototype, "computed", null);