fix: allow undefined and null values for #each in Svelte 5 (#2863)

* fix: allow `undefined` and `null` values for `#each` in Svelte 5 * switch

Author: dummdidumm

.changeset/short-pugs-kick.md

@@ -0,0 +1,6 @@
+---
+'svelte-check': patch
+'svelte2tsx': patch
+---
+
+fix: allow `undefined` and `null` values for `#each` in Svelte 5

packages/language-server/test/plugins/typescript/features/diagnostics/fixtures/each/expected_svelte_4.json

@@ -32,5 +32,22 @@
  "message": "Argument of type 'number' is not assignable to parameter of type 'ArrayLike<unknown> | Iterable<unknown>'.",
  "code": 2345,
  "tags": []
+ },
+ {
+ "range": {
+ "start": {
+ "line": 38,
+ "character": 7
+ },
+ "end": {
+ "line": 38,
+ "character": 21
+ }
+ },
+ "severity": 1,
+ "source": "ts",
+ "message": "Argument of type 'number[] | null | undefined' is not assignable to parameter of type 'ArrayLike<unknown> | Iterable<unknown>'.\n Type 'undefined' is not assignable to type 'ArrayLike<unknown> | Iterable<unknown>'.",
+ "code": 2345,
+ "tags": []
  }
 ]

packages/language-server/test/plugins/typescript/features/diagnostics/fixtures/each/expected_svelte_5.json

@@ -12,7 +12,7 @@
  },
  "severity": 1,
  "source": "ts",
- "message": "Argument of type '{}' is not assignable to parameter of type 'ArrayLike<unknown> | Iterable<unknown>'.",
+ "message": "Argument of type '{}' is not assignable to parameter of type 'ArrayLike<unknown> | Iterable<unknown> | null | undefined'.",
  "code": 2345,
  "tags": []
  },
@@ -29,7 +29,7 @@
  },
  "severity": 1,
  "source": "ts",
- "message": "Argument of type 'number' is not assignable to parameter of type 'ArrayLike<unknown> | Iterable<unknown>'.",
+ "message": "Argument of type 'number' is not assignable to parameter of type 'ArrayLike<unknown> | Iterable<unknown> | null | undefined'.",
  "code": 2345,
  "tags": []
  }

packages/language-server/test/plugins/typescript/features/diagnostics/fixtures/each/input.svelte

@@ -6,7 +6,7 @@ interface OptionObject {
 
 const simpleOptions: number[] = [];
 const complexOptions: string[] | OptionObject[] = [];
-
+const maybeUndefined: number[] | undefined | null = null as any;
 const badOptions = {
  object: {},
  number: 1,
@@ -34,3 +34,8 @@ const badOptions = {
 {#each badOptions.number as option, i}
  <div>{option} {i}</div>
 {/each}
+
+<!-- Fine in Svelte 5, error in Svelte 4 -->
+{#each maybeUndefined as option, i (i)}
+ <div>{option}, {i}</div>
+{/each}

packages/svelte2tsx/svelte-shims-v4.d.ts

@@ -250,7 +250,10 @@ declare function __sveltets_2_ensureComponent<
  : never
 >;
 
-declare function __sveltets_2_ensureArray<T extends ArrayLike<unknown> | Iterable<unknown>>(array: T): T extends ArrayLike<infer U> ? U[] : T extends Iterable<infer U> ? Iterable<U> : any[];
+declare function __sveltets_2_ensureArray<T extends ArrayLike<unknown> | Iterable<unknown>>(
+ // Svelte 5 allows undefined or null here, Svelte 4 doesn't
+ array: T | (typeof import('svelte') extends { mount: any } ? (undefined | null) : never)
+): T extends ArrayLike<infer U> ? U[] : T extends Iterable<infer U> ? Iterable<U> : any[];
 
 type __sveltets_2_PropsWithChildren<Props, Slots> = Props &
  (Slots extends { default: any }