better error message for unclose tag due to autoclosing

src/compiler/parse/index.ts

@@ -8,6 +8,12 @@ import error from '../utils/error';
 
 type ParserState = (parser: Parser) => (ParserState | void);
 
+interface LastAutoCloseTag {
+	tag: string;
+	reason: string;
+	depth: number;
+}
+
 export class Parser {
 	readonly template: string;
 	readonly filename?: string;
@@ -20,6 +26,7 @@ export class Parser {
 	css: Style[] = [];
 	js: Script[] = [];
 	meta_tags = {};
+	last_auto_closed_tag?: LastAutoCloseTag;
 
 	constructor(template: string, options: ParserOptions) {
 		if (typeof template !== 'string') {

src/compiler/parse/state/tag.ts

@@ -131,6 +131,13 @@ export default function tag(parser: Parser) {
 
 		parser.eat('>', true);
 
+		if (parser.last_auto_closed_tag && parser.last_auto_closed_tag.tag === name) {
+			parser.error({
+				code: `invalid-closing-tag`,
+				message: `<${parser.last_auto_closed_tag.tag}> cannot have child element <${parser.last_auto_closed_tag.reason}>`,
+			}, start);
+		}
+
 		// close any elements that don't have their own closing tags, e.g. <div><p></div>
 		while (parent.name !== name) {
 			if (parent.type !== 'Element')
@@ -148,10 +155,19 @@ export default function tag(parser: Parser) {
 		parent.end = parser.index;
 		parser.stack.pop();
 
+		if (parser.last_auto_closed_tag && parser.stack.length < parser.last_auto_closed_tag.depth) {
+			parser.last_auto_closed_tag = null;
+		}
+
 		return;
 	} else if (closing_tag_omitted(parent.name, name)) {
 		parent.end = start;
 		parser.stack.pop();
+		parser.last_auto_closed_tag ={
+			tag: parent.name,
+			reason: name,
+			depth: parser.stack.length,
+		};
 	}
 
 	const unique_names: Set<string> = new Set();

test/parser/samples/error-unmatched-closing-tag-autoclose/error.json

@@ -0,0 +1,10 @@
+{
+	"code": "invalid-closing-tag",
+	"message": "<p> cannot have child element <pre>",
+	"pos": 24,
+	"start": {
+		"character": 24,
+		"column": 0,
+		"line": 3
+	}
+}

test/parser/samples/error-unmatched-closing-tag-autoclose/input.svelte

@@ -0,0 +1,3 @@
+<p>
+	<pre>pre tag</pre>
+</p>