Generated type `ActionData`, is missing optional properties when `fail()` returns different `data`-shapes

Describe the bug

This is only an issue with the generated ActionData-type, not the hydration of the form-property.

When an action is calling fail() multiple places, with different shapes of data, it seems that the generated type will be narrowed down to the shape, that only shares the common properties of all the possible shapes.

This results to Property 'x' does not exist on type 'y'-errors, when trying to access those properties in the +page.svelte component

This seems like a bug. Though not of high severance, as it can be circumvented, by just adding the properties to data-objects where needed.

Related:

• #8865
• #8877

Reproduction

https://github.com/jeppech/incorrect-actiondata-multiple-fail-fn

• src/routes/+page.server.ts (named actions)
• src/routes/default/+page.server.ts (default action)

This is a plain create-svelte@latest-repo, that demonstrates how the generated ActionData-type is missing optional properties.

When the data-parameter of fail(), partially matches other shapes, in any other call to the fail-function, the generated type will be narrowed down to the "common" properties of all shapes.

Logs

No response

System Info

❯ pnpx envinfo --system --binaries --browsers --npmPackages "{svelte,@sveltejs/*,vite}"
.../../.node/pnpm/store/v3/tmp/dlx-55577 |   +1 +
.../../.node/pnpm/store/v3/tmp/dlx-55577 | Progress: resolved 1, reused 1, downloaded 0, added 1, done

  System:
    OS: macOS 13.3.1
    CPU: (8) arm64 Apple M1 Pro
    Memory: 237.09 MB / 16.00 GB
    Shell: 5.9 - /bin/zsh
  Binaries:
    Node: 18.12.1 - ~/Library/pnpm/node
    npm: 8.18.0 - /opt/homebrew/bin/npm
  Browsers:
    Brave Browser: 112.1.50.119
    Firefox Developer Edition: 113.0
    Safari: 16.4
  npmPackages:
    @sveltejs/adapter-auto: ^2.0.0 => 2.0.0
    @sveltejs/kit: ^1.15.7 => 1.15.7
    svelte: ^3.58.0 => 3.58.0
    vite: ^4.2.2 => 4.2.2

Severity

annoyance

Additional Information

No response

Start with the reproduction files src/routes/+page.server.ts and src/routes/default/+page.server.ts, then inspect how their multiple fail() data shapes produce the generated ActionData type used by +page.svelte. Done means optional properties from the possible data shapes remain accessible without type errors in the page component.