Svelte 5: Cannot specify this' type in TypeScript

[Florob]

Describe the bug

When using TypeScript the syntax for specifying the type of this is no longer supported. Builds throw a syntax error. Dev server removes the type, but keeps the this parameter, leading to syntax errors at runtime.

Reproduction

function onclick(this: HTMLButtonElement, event: Event) 

REPL

Logs

vite build:
✓ 204 modules transformed.
x Build failed in 3.56s
error during build:
RollupError: Expected ident
    at getRollupError (file://.../node_modules/rollup/dist/es/shared/parseAst.js:396:41)
    at ParseError.initialise (file://.../node_modules/rollup/dist/es/shared/node-entry.js:11332:28)
    at convertNode (file://.../node_modules/rollup/dist/es/shared/node-entry.js:13082:10)
    at convertProgram (file://.../node_modules/rollup/dist/es/shared/node-entry.js:12399:12)
    at Module.setSource (file://.../node_modules/rollup/dist/es/shared/node-entry.js:14246:24)
    at async ModuleLoader.addModuleSource (file://.../node_modules/rollup/dist/es/shared/node-entry.js:18892:13)

Browser via vite:
Uncaught SyntaxError: missing formal parameter

System Info

System:
    OS: Linux 6.9 Arch Linux
    CPU: (8) x64 Intel(R) Core(TM) i7-3720QM CPU @ 2.60GHz
    Memory: 9.32 GB / 15.20 GB
    Container: Yes
    Shell: 5.9 - /bin/zsh
  Binaries:
    Node: 22.2.0 - /usr/bin/node
    Yarn: 1.22.22 - /usr/bin/yarn
    npm: 10.8.0 - /usr/bin/npm
  Browsers:
    Chromium: 125.0.6422.60
  npmPackages:
    svelte: ^5.0.0-next.138 => 5.0.0-next.138 
    webpack: ^5.64.1 => 5.91.0

Severity

blocking an upgrade

[dummdidumm]

related to #11502, though I wonder if we should treat this instance specifically

[paoloricciuti]

related to #11502, though I wonder if we should treat this instance specifically

Well this is more of a typing issue rather than a runtime one right? I would say if we can we should try to make this work.

[Rich-Harris]

yeah, we should definitely fix this