fix: avoid erroneous async derived expressions for blocks #17604
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fixes #17595 When an if/key/etc block has an expression that depends on an async blocker (e.g., is inside a component with top level `await`), the compiler incorrectly treats the expression as async - even when the expression itself contains no `await`. This causes the expression to be added to `$.async`'s `expressions` array, which wraps it in an `async_derived`. This is not only unnecessary but also buggy: it breaks the direct reactive connection between the source and its dependent effects, causing inconsistent effect executions. The fix is to only add expressions to `$.async`'s `expressions` array when they actually contain an `await`.
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* fix: reschedule effects inside unskipped branches When a branch is speculatively marked for destruction (condition temporarily falsy), its child effects are reset to `CLEAN` to prevent them running in a doomed branch (as of #17581). However, if the branch survives (condition becomes truthy again), those effects remain `CLEAN` and never run - the source was already marked dirty before the reset, so no new dirty marking occurs. The fix is to change `skipped_effects` from a `Set` to a `Map` that tracks which child effects were dirty/maybe_dirty before being reset. When a branch is unskipped (survives), restore their status and reschedule them. * update comment * lint * make skipped_effects private * actually while we're at it let's use a more descriptive name * prettier --------- Co-authored-by: Rich Harris <[email protected]>
Rich-Harris
approved these changes
Feb 4, 2026
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fixes #17595
When an if/key/etc block has an expression that depends on an async blocker (e.g., is inside a component with top level
await), the compiler incorrectly treats the expression as async - even when the expression itself contains noawait.This causes the expression to be added to
$.async'sexpressionsarray, which wraps it in anasync_derived. This is not only unnecessary but also buggy: it breaks the direct reactive connection between the source and its dependent effects, causing inconsistent effect executions.The fix is to only add expressions to
$.async'sexpressionsarray when they actually contain anawait....however, you'll notice that for now this will fail one test. This is due to an unrelated bug which is fixed by #17605 (separate PR merging into this one for easier review).
Before submitting the PR, please make sure you do the following
feat:,fix:,chore:, ordocs:.packages/svelte/src, add a changeset (npx changeset).Tests and linting
pnpm testand lint the project withpnpm lint