Breaking Change Regression: Svelte fails to parse non-null assert in functions

[AlbertMarashi]

Describe the bug

Svelte fails to parse a non-null assertion within a function has a return type annotation

Reproduction

<script lang="ts">
function foo(): string {
    return ""!
}
</script>

// works
function foo(): string {
    return ""
}

// works
function foo() {
    return ""!
}

// fails
function foo(): string {
    return ""!
}

Logs

error during build:
[vite-plugin-svelte] [plugin vite-plugin-svelte] Cannot read properties of undefined (reading 'startsWith')
file: /Users/albert/svelte-bug-3/src/routes/+page.svelte
TypeError: [plugin vite-plugin-svelte] Cannot read properties of undefined (reading 'startsWith')
    at couldBeFixedByCssPreprocessor (file:///Users/albert/svelte-bug-3/node_modules/@sveltejs/vite-plugin-svelte/src/utils/error.js:111:72)
    at enhanceCompileError (file:///Users/albert/svelte-bug-3/node_modules/@sveltejs/vite-plugin-svelte/src/utils/error.js:126:6)
    at compileSvelte (file:///Users/albert/svelte-bug-3/node_modules/@sveltejs/vite-plugin-svelte/src/utils/compile.js:148:4)
    at async Object.transform (file:///Users/albert/svelte-bug-3/node_modules/@sveltejs/vite-plugin-svelte/src/index.js:164:20)
    at async transform (file:///Users/albert/svelte-bug-3/node_modules/rollup/dist/es/shared/node-entry.js:19663:16)
    at async ModuleLoader.addModuleSource (file:///Users/albert/svelte-bug-3/node_modules/rollup/dist/es/shared/node-entry.js:19879:36)
error: script "build" exited with code 1

System Info

System:
    OS: macOS 15.1
    CPU: (8) arm64 Apple M1 Pro
    Memory: 84.80 MB / 16.00 GB
    Shell: 5.9 - /bin/zsh
  Binaries:
    Node: 20.17.0 - ~/.nvm/versions/node/v20.17.0/bin/node
    npm: 10.8.2 - ~/.nvm/versions/node/v20.17.0/bin/npm
    pnpm: 9.7.0 - ~/Library/pnpm/pnpm
    bun: 1.1.39 - ~/.bun/bin/bun
  Browsers:
    Chrome: 131.0.6778.140
    Safari: 18.1
  npmPackages:
    svelte: ^5.14.3 => 5.14.3

Severity

blocking all usage of svelte

[AlbertMarashi]

@Rich-Harris related

• TypeScript nodes still causing code generation failures #14743

[xeho91]

Another hotfix that may potentially solve this issue hasn't been released yet. You can track progress here: #14745

If you're inpatient, there's a workaround: #14741 (comment)

[dummdidumm]

Should be fixed now