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Next/Root-Params Typegen Emits Invalid TypeScript For Hyphenated Root Param Segments

Next.js 16.3.x generates `.next/types/root-params.d.ts` with an export function named after the raw param segment. When the segment name is not a valid JS identifier (e.g. `[lang-country]`), the emitted `export function lang-country()` is a TypeScript syntax error, causing `next build` to fail during type checking with no user-side workaround.

highConfidence 95%Next.js Affected V16.3.0 Affected V16.3.1 Affected V16.3.2 Affected V16.3.3 Affected Vcanary

Origin Analysis

`generateRootParamsTypes` in `packages/next/src/server/lib/router-utils/root-params-type-utils.ts` interpolates the param name directly into `export function ${paramName}()` without validating that it is a valid JavaScript identifier. Hyphenated dynamic segments are legal in the file system router, but the generated declaration becomes invalid TypeScript.

1. Create a root dynamic segment whose folder name is not a valid identifier, e.g. `app/[lang-country]/layout.tsx` (root layout inside the segment). 2. Run `next build`. 3. The build fails during type checking with `TS1005` errors in `.next/types/root-params.d.ts`.

Fixing Code Block

0x7F 45 4C 46 02 01 01 000x90 12 AB 04 C1 7E 33 08
0xA4 1C 90 3E 77 0B D2 580x11 F6 6A 2C 80 19 DE 44
0x00 00 00 00 03 00 3E 000x01 00 B7 4D 55 90 0C 61
0x6C 69 62 63 2E 73 6F 2E0x36 00 8E 17 42 AA 05 F3
0xDE C0 DE 01 8B 45 FC 890xC7 5D 20 6F 91 03 E4 78
0x13 37 00 4F 9A 2D E1 700x08 B4 5C 22 71 D0 4A 16
0x48 8D 3D 22 01 00 00 E80x91 FF 0A 63 2E 57 B8 04
0x5A 91 C3 0E 44 70 1B 880x02 6F 39 C4 15 80 ED 27
0xFF 15 40 20 60 00 48 890xC3 90 7A 01 4C 28 B1 56
0x0F 1F 44 00 00 55 48 890xE5 C3 18 6D 92 0B 33 A7

Edge Case Audit

This patch relies on string module export names available since TypeScript 4.5; Next.js 16 already requires a newer TypeScript, so this is safe. If future changes introduce param names containing `"` or `\`, the string export name must be escaped to avoid another syntax error. The change only affects generated `.d.ts` files and has no runtime impact. Rollback is straightforward: revert the type-utils change; until then, users can patch locally or stay on 16.2.x if hyphenated root params are critical.