error on empty name in `class:` directive (#5939)

src/compiler/parse/state/tag.ts

@@ -387,6 +387,13 @@ function read_attribute(parser: Parser, unique_names: Set<string>) {
 			}, start);
 		}
 
+		if (type === 'Class' && directive_name === '') {
+			parser.error({
+				code: 'invalid-class-directive',
+				message: 'Class binding name cannot be empty'
+			}, start + colon_index + 1);
+		}
+
 		if (value[0]) {
 			if ((value as any[]).length > 1 || value[0].type === 'Text') {
 				parser.error({

test/parser/samples/error-empty-classname-binding/error.json

@@ -0,0 +1,10 @@
+{
+	"code": "invalid-class-directive",
+	"message": "Class binding name cannot be empty",
+	"start": {
+		"line": 1,
+		"column": 10,
+		"character": 10
+	},
+	"pos": 10
+}

test/parser/samples/error-empty-classname-binding/input.svelte

@@ -0,0 +1 @@
+<h1 class:={true}>Hello</h1>