fix(compiler-core): adjacent v-else should cause a compiler error (#13699)

close #13698

Author: linzhe141

packages/compiler-core/__tests__/transforms/vIf.spec.ts

@@ -301,6 +301,25 @@ describe('compiler: v-if', () => {
       ])
     })
 
+    test('error on adjacent v-else', () => {
+      const onError = vi.fn()
+
+      const {
+        node: { branches },
+      } = parseWithIfTransform(
+        `<div v-if="false"/><div v-else/><div v-else/>`,
+        { onError },
+        0,
+      )
+
+      expect(onError.mock.calls[0]).toMatchObject([
+        {
+          code: ErrorCodes.X_V_ELSE_NO_ADJACENT_IF,
+          loc: branches[branches.length - 1].loc,
+        },
+      ])
+    })
+
     test('error on user key', () => {
       const onError = vi.fn()
       // dynamic

packages/compiler-core/src/transforms/vIf.ts

@@ -141,9 +141,9 @@ export function processIf(
       }
 
       if (sibling && sibling.type === NodeTypes.IF) {
-        // Check if v-else was followed by v-else-if
+        // Check if v-else was followed by v-else-if or there are two adjacent v-else
         if (
-          dir.name === 'else-if' &&
+          (dir.name === 'else-if' || dir.name === 'else') &&
           sibling.branches[sibling.branches.length - 1].condition === undefined
         ) {
           context.onError(