fix: avoid false duplicate route warning for named views

Closes #2648

Author: posva

packages/router/src/unplugin/core/tree.spec.ts

@@ -1275,6 +1275,14 @@ describe('Tree', () => {
       expect(c.children.has('_parent')).toBe(false)
     })
 
+    it('does not flag named views as conflicts', () => {
+      const tree = new PrefixTree(RESOLVED_OPTIONS)
+      tree.insert('posts/index', 'posts/index.vue')
+      tree.insert('posts/index@header', 'posts/index@header.vue')
+
+      expect(collectDuplicatedRouteNodes(tree)).toEqual([])
+    })
+
     it('collects _parent conflicts', () => {
       const tree = new PrefixTree(RESOLVED_OPTIONS)
       const nested = tree.insert('nested', 'nested.vue')

packages/router/src/unplugin/core/tree.ts

@@ -624,7 +624,12 @@ export function collectDuplicatedRouteNodes(
   }
 
   const dups = Array.from(seen.values())
-    .filter(nodes => Object.keys(nodes).length > 1)
+    // All entries in a group reference the same TreeNode instance, so
+    // comparing the number of files to components.size tells us if any
+    // file was overwritten (e.g. index.vue vs index@default.vue both
+    // targeting the "default" view). Different named views on the same
+    // node (e.g. index.vue + index@header.vue) are not conflicts.
+    .filter(nodes => nodes.length > nodes[0].node.value.components.size)
     .map(nodes =>
       nodes.toSorted(({ node: a }, { node: b }) => {
         // put the one that takes precedence at the end of the list