Gate the webpack opt-out at Next >= 16, not >= 15

`next build --webpack` was added in Next 16 (when Turbopack became the build
default); Next 15 builds with webpack already and errors on the unknown flag,
which would break `mailing server build` for Next 15 users. Same for the
programmatic dev server's `webpack: true` (only needed on Next 16, whose
dev server defaults to Turbopack). Threshold corrected 15 -> 16.

Author: psugihara

packages/cli/src/commands/preview/server/start.ts

@@ -55,9 +55,9 @@ export default async function startPreviewServer() {
 
   // Next 16 defaults the dev server to Turbopack, which can't resolve some of
   // the preview app's dynamic dependencies (e.g. uglify-js) and returns 500s.
-  // Force webpack on Next >= 15, where a custom server may pass this option;
-  // older Next uses webpack by default. See gh#504.
-  if (nextMajorVersion() >= 15) nextOptions.webpack = true;
+  // Force webpack on Next >= 16, where a custom server may pass this option;
+  // Next <= 15 uses webpack by default and ignores the option. See gh#504.
+  if (nextMajorVersion() >= 16) nextOptions.webpack = true;
 
   const app = next(nextOptions);
   const nextHandle = app.getRequestHandler();

packages/cli/src/commands/server.ts

@@ -42,10 +42,11 @@ export const builder = {
 // Next 16 makes `next build` use Turbopack by default, whose static analysis
 // can't follow the .mailing preview app's dynamic requires (e.g. the generated
 // Prisma client), so the build fails ("Encountered unexpected file in NFT
-// list" / EBADF). Opt back into webpack on Next >= 15, where the `--webpack`
-// flag exists; older Next versions build with webpack by default. See gh#504.
+// list" / EBADF). Opt back into webpack on Next >= 16, where the `--webpack`
+// flag was added; Next <= 15 builds with webpack by default and errors on the
+// unknown flag. See gh#504.
 function nextBuildBundlerFlag(): string {
-  return nextMajorVersion() >= 15 ? " --webpack" : "";
+  return nextMajorVersion() >= 16 ? " --webpack" : "";
 }
 
 export const handler = buildHandler(async (argv: ServerArguments) => {