fix: allow transitions to return a function taking a direction argument#3081
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Nic-Polumeyv wants to merge 1 commit into
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fix: allow transitions to return a function taking a direction argument#3081Nic-Polumeyv wants to merge 1 commit into
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Custom transitions may return `(options: { direction: 'in' | 'out' }) => TransitionConfig`,
which Svelte supports, but the `__sveltets_2_SvelteTransitionReturnType` shim only allowed
`TransitionConfig | (() => TransitionConfig)`. A function with a required parameter is not
assignable to `() => TransitionConfig`, so valid custom transitions were reported as errors.
Widen the shim's return type to match Svelte's, in both the v4 and v3 shims. The added
transition-options fixture fails without the change.
Fixes sveltejs#2686
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Fixes #2686.
Custom transitions may return a function taking a
directionargument — Svelte always calls the deferred config with{ direction: 'in' | 'out' }(Svelte 5, Svelte 4) — but the__sveltets_2_SvelteTransitionReturnTypeshim only allowedTransitionConfig | (() => TransitionConfig). A function with a required parameter isn't assignable to() => TransitionConfig, so valid custom transitions were flagged as errors while the parameterless form was accepted.This widens the shim's deferred form to
(options: { direction: 'in' | 'out' }) => TransitionConfig, the same type Svelte 5's runtime uses internally. Note that the publicTransitionFntype marksdirectionoptional instead; the required form here is deliberate. Because function parameters are checked contravariantly, the required form accepts every shape users can write —() => TransitionConfigas well as callbacks declaringdirectionrequired, optional, or widened to'in' | 'out' | 'both'— whereas an optional-directionshim would still reject the issue's reproduction understrictFunctionTypes(a{ direction?: 'in' | 'out' }argument isn't assignable to a parameter typed{ direction: 'in' | 'out' }). Existing code is unaffected since() => TransitionConfigremains assignable.I applied the same change to
svelte-shims.d.tssince the v4 shim's header asks to consider backporting changes; happy to drop that hunk if you'd rather leave the Svelte-3-only shim untouched.The
transition-optionsdiagnostics fixture is extended with a transition returning adirection-taking function; it fails without the shim change and passes with it.