Fix SWC decorator parsing to support TS5 stage-3 decorators

[arab971]

What?

Fix SWC decorator parsing to support TS5 stage-3 decorators. Currently, SWC rejects @decorator syntax at the parser level for TypeScript files unless experimentalDecorators is set in tsconfig, making TS5 stage-3 decorators impossible.

Why?

TypeScript 5 introduced stage-3 decorators as the default (no flag needed). Next.js users adopting TS5 decorators hit a parse error (Unexpected token '@') because SWC's parser only enabled decorator syntax when experimentalDecorators: true was set. This is the #1 blocker reported in #48360.

How?

• Parser (swc/options.ts): Always enable decorator parsing for .ts/.tsx files, regardless of experimentalDecorators
• Transform (swc/options.ts): Use decoratorVersion: "2022-03" (stage-3 Ecma proposal) when experimentalDecorators is not set, matching Turbopack's DecoratorsKind::Ecma behavior
• Webpack (webpack-config.ts): Set decorators: true unconditionally; gate emitDecoratorMetadata on experimentalDecorators (metadata is a legacy-only feature)

Fixes #48360

Test plan

• Integration test added: test/production/ts5-decorators/ — builds a page using TS5 decorators and verifies it renders
• Verify pnpm test-start-webpack test/production/ts5-decorators/index.test.ts passes

Manual verification with issue reproduction app

# 1. Build local Next.js
pnpm --filter=next build

# 2. Download the reproduction app from #48360, then update dependencies:
cd <reproduction-app>
# In package.json, set:
#   "next": "link:<path-to-local-next.js>/packages/next"
#   "react": "^19.0.0", "react-dom": "^19.0.0"

# 3. Build with webpack (Turbopack has separate workspace root issue)
rm -f yarn.lock package-lock.json && pnpm install
npx next build --webpack

# Expected: ✓ Compiled successfully, decorator logs visible in output

[kdy1]

2022-03 sounds wrong

[arab971]

@kdy1 I completely understand why that looks off—it looks like a date! However, 2022-03 is the specific version string required by the SWC compiler to trigger the Stage 3 (EcmaScript) decorator transformation. It's an internal SWC convention rather than a Next.js one. I've aligned this with Turbopack's DecoratorsKind::Ecma to ensure we have consistent behavior across both bundlers.

[kdy1]

Please do not ping me

[TechQuery]

Please do not ping me

What happened here? Why is this PR closed without reviewing? Why cannot contributers ping reviewers?

TS 7 has already been RC version, Next.js hasn't fully supported TS 5 ?!

Support custom SWC configuration just like Babel, or merge this PR please!

[arab971]

@TechQuery Thank you for your support. I apologize for closing the PR; I was feeling frustrated by the response from the reviewer, as well as the general lack of feedback and reviews over such a long period. I truly appreciate you bringing this to my attention.

[TechQuery]

@TechQuery Thank you for your support. I apologize for closing the PR; I was feeling frustrated by the response from the reviewer, as well as the general lack of feedback and reviews over such a long period. I truly appreciate you bringing this to my attention.

You're welcome, Next.js mainteners are the people who should be blamed. I had similar situation in fixing the broken Vercel serverless template: vercel/examples#835

I think, maybe Vercel is humanless now. If I'm not too busy in the future, I will finish my SSR framework based on Web components standard & fully customable SWC compiler: EasyWebApp/WebSSR#1

[arab971]

@lubieowoce — this PR fixes TS5/TS7 stage-3 decorator support in SWC builds. kdy1 confirmed the SWC convention is correct. Can you take a look?