Await uses the previous value when promise is updated

[AndersRobstad]

Describe the bug

Having a promise that is stored in a state rune and awaited in the template code seems to have a bug where it uses the previously resolved value of the promise to render the template. The issue appeared in our codebase in a place where we have a promise returning data that can have different shapes. When the promise variable is reassigned a new promise our application crashes due to the code trying to access a value which is no longer available due to the variable having a new promise as its state.

I would have expected the re-assigning the variable containing the promise would instantly render the loading state such that the code inside the {then} block would not be hit until the new promise has resolved. The example is quite simplified in the REPL but it shows the general problem.

So to reproduce:

• Have a state rune with promise that can return different shapes of data
• await the promise in the template and access the resolved value
• Re-assign the state variable and see that the application crashes

Reproduction

Click the button and see that the app crashes due to accessing a value which no longer exists since the promise has been updated to a new promise which should render the loading state instead of the data.

Repro: https://svelte.dev/playground/0515270734184612838039274f00acde?version=5.38.6

Logs

See the log in the REPL when clicking the button

System Info

System:
    OS: macOS 15.5
    CPU: (10) arm64 Apple M2 Pro
    Memory: 148.02 MB / 16.00 GB
    Shell: 5.9 - /bin/zsh
  Binaries:
    Node: 22.15.0 - ~/.nvm/versions/node/v22.15.0/bin/node
    npm: 10.9.2 - ~/.nvm/versions/node/v22.15.0/bin/npm
    pnpm: 10.14.0 - ~/Library/pnpm/pnpm
  Browsers:
    Brave Browser: 137.1.79.118
    Chrome: 139.0.7258.155
    Edge: 139.0.3405.125
    Safari: 18.5

Severity

annoyance

[MotionlessTrain]

It is not caused by the await block, but rather by calls
In the repl, calls is updated when the button is clicked, synchronously.
So the if block within the {:then} is updated immediately, and tries to access data.second before the {#await} even has time to go back to loading again. If the calls++ statement is moved to the end of the timeout within the fetching function, it works as expected