disallow duplicate slot="xxx" elements - fixes second half of #2320

src/compile/Component.ts

@@ -119,6 +119,7 @@ export default class Component {
 	globally_used_names: Set<string> = new Set();
 
 	slots: Map<string, Slot> = new Map();
+	slot_outlets: Set<string> = new Set();
 
 	constructor(
 		ast: Ast,

src/compile/nodes/Element.ts

@@ -361,6 +361,15 @@ export default class Element extends Node {
 					});
 				}
 
+				if (component.slot_outlets.has(name)) {
+					component.error(attribute, {
+						code: `duplicate-slot-attribute`,
+						message: `Duplicate '${name}' slot`
+					});
+
+					component.slot_outlets.add(name);
+				}
+
 				let ancestor = this.parent;
 				do {
 					if (ancestor.type === 'InlineComponent') break;

src/compile/nodes/Slot.ts

@@ -40,15 +40,6 @@ export default class Slot extends Element {
 			}
 
 			this.values.set(attr.name, new Attribute(component, this, scope, attr));
-
-			// TODO should duplicate slots be disallowed? Feels like it's more likely to be a
-			// bug than anything. Perhaps it should be a warning
-
-			// if (validator.slots.has(slot_name)) {
-			// 	validator.error(`duplicate '${slot_name}' <slot> element`, nameAttribute.start);
-			// }
-
-			// validator.slots.add(slot_name);
 		});
 
 		if (!this.slot_name) this.slot_name = 'default';
@@ -76,12 +67,5 @@ export default class Slot extends Element {
 		}
 
 		component.slots.set(this.slot_name, this);
-
-		// if (node.attributes.length === 0) && validator.slots.has('default')) {
-		// 	validator.error(node, {
-		// 		code: `duplicate-slot`,
-		// 		message: `duplicate default <slot> element`
-		// 	});
-		// }
 	}
 }