fix: error if `{:else}` is followed by another `{:else}` or `{:else if ...}` (#18900)

Fixes #18202

.changeset/duplicate-else-clause.md

@@ -0,0 +1,5 @@
+---
+'svelte': patch
+---
+
+fix: error if `{:else}` is followed by another `{:else}` or `{:else if ...}`

packages/svelte/src/compiler/phases/1-parse/state/tag.js

@@ -532,6 +532,7 @@ function next(parser) {
 	if (block.type === 'IfBlock') {
 		if (!parser.eat('else')) e.expected_token(start, '{:else} or {:else if}');
 		if (parser.eat('if')) e.block_invalid_elseif(start);
+		if (block.alternate) e.block_duplicate_clause(start, '{:else}');
 
 		parser.allow_whitespace();
 
@@ -580,6 +581,7 @@ function next(parser) {
 
 	if (block.type === 'EachBlock') {
 		if (!parser.eat('else')) e.expected_token(start, '{:else}');
+		if (block.fallback) e.block_duplicate_clause(start, '{:else}');
 
 		parser.allow_whitespace();
 		parser.eat('}', true);

packages/svelte/tests/compiler-errors/samples/each-block-duplicate-else/_config.js

@@ -0,0 +1,9 @@
+import { test } from '../../test';
+
+export default test({
+	error: {
+		code: 'block_duplicate_clause',
+		message: '{:else} cannot appear more than once within a block',
+		position: [42, 42]
+	}
+});

packages/svelte/tests/compiler-errors/samples/each-block-duplicate-else/main.svelte

@@ -0,0 +1,7 @@
+{#each items as item}
+	{item}
+{:else}
+	A
+{:else}
+	B
+{/each}

packages/svelte/tests/compiler-errors/samples/if-block-else-before-else-if/_config.js

@@ -0,0 +1,9 @@
+import { test } from '../../test';
+
+export default test({
+	error: {
+		code: 'block_duplicate_clause',
+		message: '{:else} cannot appear more than once within a block',
+		position: [23, 23]
+	}
+});

packages/svelte/tests/compiler-errors/samples/if-block-else-before-else-if/main.svelte

@@ -0,0 +1,7 @@
+{#if a}
+	A
+{:else}
+	B
+{:else if b}
+	C
+{/if}