fail validation if bound <select> has dynamic multiple attribute - fixes #1270

src/validate/html/validateElement.ts

@@ -102,7 +102,20 @@ export default function validateElement(
 					);
 				}
 
+				if (node.name === 'select') {
+					const attribute = node.attributes.find(
+						(attribute: Node) => attribute.name === 'multiple'
+					);
+
+					if (attribute && isDynamic(attribute)) {
+						validator.error(
+							`'multiple' attribute cannot be dynamic if select uses two-way binding`,
+							attribute
+						);
+					}
+				} else {
 					checkTypeAttribute(validator, node);
+				}
 			} else if (name === 'checked' || name === 'indeterminate') {
 				if (node.name !== 'input') {
 					validator.error(

test/validator/samples/binding-select-multiple-dynamic/errors.json

@@ -0,0 +1,12 @@
+[{
+	"message": "'multiple' attribute cannot be dynamic if select uses two-way binding",
+	"loc": {
+		"line": 1,
+		"column": 19
+	},
+	"end": {
+		"line": 1,
+		"column": 28
+	},
+	"pos": 19
+}]

test/validator/samples/binding-select-multiple-dynamic/input.html

@@ -0,0 +1,5 @@
+<select bind:value :multiple>
+	<option>1</option>
+	<option>2</option>
+	<option>3</option>
+</select>