fix: Allow @const inside #key (#15377)

Author: michelAlexis

.changeset/healthy-guests-itch.md

@@ -0,0 +1,5 @@
+---
+'svelte': patch
+---
+
+Allow `@const` inside `#key`

packages/svelte/src/compiler/phases/2-analyze/visitors/ConstTag.js

@@ -25,6 +25,7 @@ export function ConstTag(node, context) {
 			grand_parent?.type !== 'AwaitBlock' &&
 			grand_parent?.type !== 'SnippetBlock' &&
 			grand_parent?.type !== 'SvelteBoundary' &&
+			grand_parent?.type !== 'KeyBlock' &&
 			((grand_parent?.type !== 'RegularElement' && grand_parent?.type !== 'SvelteElement') ||
 				!grand_parent.attributes.some((a) => a.type === 'Attribute' && a.name === 'slot')))
 	) {

packages/svelte/tests/validator/samples/const-tag-inside-key-block/errors.json

@@ -0,0 +1 @@
+[]

packages/svelte/tests/validator/samples/const-tag-inside-key-block/input.svelte

@@ -0,0 +1,3 @@
+{#key 'key'}
+	{@const foo = 'bar'}
+{/key}