fix: handle empty else if block in legacy AST (#12397)

Author: jasonlyu123

.changeset/tall-dragons-switch.md

@@ -0,0 +1,5 @@
+---
+'svelte': patch
+---
+
+fix: handle empty else if block in legacy AST

packages/svelte/src/compiler/legacy.js

@@ -353,7 +353,11 @@ export function convert(source, ast) {
 					};
 				}
 
-				const start = node.elseif ? node.consequent.nodes[0].start : node.start;
+				const start = node.elseif
+					? node.consequent.nodes[0]?.start ??
+						source.lastIndexOf('{', /** @type {number} */ (node.end) - 1)
+					: node.start;
+
 				remove_surrounding_whitespace_nodes(node.consequent.nodes);
 
 				return {

packages/svelte/tests/parser-legacy/samples/if-block-elseif/input.svelte

@@ -2,4 +2,4 @@
 	<p>x is greater than 10</p>
 {:else if x < 5}
 	<p>x is less than 5</p>
-{/if}
+{:else if x === 1}{/if}

packages/svelte/tests/parser-legacy/samples/if-block-elseif/output.json

@@ -2,12 +2,12 @@
 	"html": {
 		"type": "Fragment",
 		"start": 0,
-		"end": 89,
+		"end": 107,
 		"children": [
 			{
 				"type": "IfBlock",
 				"start": 0,
-				"end": 89,
+				"end": 107,
 				"expression": {
 					"type": "BinaryExpression",
 					"start": 5,
@@ -78,12 +78,12 @@
 				"else": {
 					"type": "ElseBlock",
 					"start": 58,
-					"end": 84,
+					"end": 102,
 					"children": [
 						{
 							"type": "IfBlock",
 							"start": 58,
-							"end": 89,
+							"end": 107,
 							"expression": {
 								"type": "BinaryExpression",
 								"start": 52,
@@ -151,6 +151,69 @@
 									]
 								}
 							],
+							"else": {
+								"type": "ElseBlock",
+								"start": 102,
+								"end": 102,
+								"children": [
+									{
+										"type": "IfBlock",
+										"start": 102,
+										"end": 107,
+										"expression": {
+											"type": "BinaryExpression",
+											"start": 94,
+											"end": 101,
+											"loc": {
+												"start": {
+													"line": 5,
+													"column": 10
+												},
+												"end": {
+													"line": 5,
+													"column": 17
+												}
+											},
+											"left": {
+												"type": "Identifier",
+												"start": 94,
+												"end": 95,
+												"loc": {
+													"start": {
+														"line": 5,
+														"column": 10
+													},
+													"end": {
+														"line": 5,
+														"column": 11
+													}
+												},
+												"name": "x"
+											},
+											"operator": "===",
+											"right": {
+												"type": "Literal",
+												"start": 100,
+												"end": 101,
+												"loc": {
+													"start": {
+														"line": 5,
+														"column": 16
+													},
+													"end": {
+														"line": 5,
+														"column": 17
+													}
+												},
+												"value": 1,
+												"raw": "1"
+											}
+										},
+										"children": [],
+										"elseif": true
+									}
+								]
+							},
 							"elseif": true
 						}
 					]