Svelte 5: Inaccurate source maps when `lang="ts"` is enabled

[AdrianGonz97]

Describe the bug

Source maps are seemingly inaccurate when lang="ts" is enabled on the <script> tag. This only occurs when using TS in Svelte 5. Removing lang="ts" fixes the issue.

When presented with the following Svelte code:

Errors locations are accurate in the IDE (as shown above), but are off in both the terminal output, and the error page.

Reproduction

https://github.com/AdrianGonz97/svelte-5-ts-source-maps-reproduction

Logs

No response

System Info

  System:
    OS: Linux 5.15 Ubuntu 20.04.5 LTS (Focal Fossa)
    CPU: (24) x64 AMD Ryzen 9 5900X 12-Core Processor
    Memory: 7.95 GB / 31.30 GB
    Container: Yes
    Shell: 5.8 - /usr/bin/zsh
  Binaries:
    Node: 20.10.0 - ~/.volta/tools/image/node/20.10.0/bin/node
    Yarn: 1.22.21 - ~/.volta/tools/image/yarn/1.22.21/bin/yarn
    npm: 10.2.3 - ~/.volta/tools/image/node/20.10.0/bin/npm
    pnpm: 8.10.5 - ~/.volta/bin/pnpm
    bun: 1.0.2 - ~/.bun/bin/bun
  npmPackages:
    svelte: ^5.0.0-next.26 => 5.0.0-next.26

Severity

annoyance

[AdrianGonz97]

Removing vitePreprocess from the preprocess array in svelte.config.js seems to also fix the source maps. Perhaps the issue is originating from there?

[dummdidumm]

Sourcemaps with TS should work as of #10459

[AdrianGonz97]

Thanks for taking a look, @dummdidumm!

Unfortunately, the issue is still present even with the latest version of Svelte 5. I've updated the dependencies in the reproduction if you'd like to check it out.

However, since messing with the preprocess field here:

Removing vitePreprocess from the preprocess array in svelte.config.js seems to also fix the source maps. Perhaps the issue is originating from there?

I'm thinking that the sourcemaps may actually be borked from the vite-plugin-svelte layer instead. Although, I'm unsure as to why the error highlighting in the IDE vs. the error window that appears on the site (as shown in images of the original issue above) are different from each other 🤔

[andreapiso]

Is there a fix for this? Our source maps are all wrong on svelte 5 right now

[VismaTobbe]

This is super annoying when debugging