Version

3.0.0-rc.9

Reproduction link

https://github.com/luka-mikec/vue-once-if-issue-demo

Steps to reproduce

1. Create a new Vue 3 project (through vue-cli or vite)
2. Replace App.vue with: https://github.com/luka-mikec/vue-once-if-issue-demo/blob/master/src/App.vue
3. Run npm run serve (or npm run dev with Vite) and try loading the app in a browser

What is expected?

The resulting webpage should display "a"

What is actually happening?

Error (vue-cli):

 ERROR  Failed to compile with 1 errors                                                                                                                                                                 2:32:19 PM

 error  in ./src/App.vue?vue&type=template&id=7ba5bd90

Module build failed (from ./node_modules/vue-loader-v16/dist/templateLoader.js):
TypeError: Cannot read property '2' of undefined
    at injectProp (/my-project-path/node_modules/@vue/compiler-core/dist/compiler-core.cjs.js:443:82)
    at createChildrenCodegenNode (/my-project-path/node_modules/@vue/compiler-core/dist/compiler-core.cjs.js:2841:9)
    at createCodegenNodeForBranch (/my-project-path/node_modules/@vue/compiler-core/dist/compiler-core.cjs.js:2797:62)
    at Array.<anonymous> (/my-project-path/node_modules/@vue/compiler-core/dist/compiler-core.cjs.js:2696:38)
    at traverseNode (/my-project-path/node_modules/@vue/compiler-core/dist/compiler-core.cjs.js:1681:19)
    at traverseChildren (/my-project-path/node_modules/@vue/compiler-core/dist/compiler-core.cjs.js:1624:9)
    at traverseNode (/my-project-path/node_modules/@vue/compiler-core/dist/compiler-core.cjs.js:1675:13)
    at traverseChildren (/my-project-path/node_modules/@vue/compiler-core/dist/compiler-core.cjs.js:1624:9)
    at traverseNode (/my-project-path/node_modules/@vue/compiler-core/dist/compiler-core.cjs.js:1675:13)
    at transform (/my-project-path/node_modules/@vue/compiler-core/dist/compiler-core.cjs.js:1566:5)
    at Object.baseCompile (/my-project-path/node_modules/@vue/compiler-core/dist/compiler-core.cjs.js:4115:5)
    at Object.compile (/my-project-path/node_modules/@vue/compiler-dom/dist/compiler-dom.cjs.js:3033:25)
    at doCompileTemplate (/my-project-path/node_modules/@vue/compiler-sfc/dist/compiler-sfc.cjs.js:580:34)
    at Object.compileTemplate (/my-project-path/node_modules/@vue/compiler-sfc/dist/compiler-sfc.cjs.js:564:16)
    at Object.TemplateLoader (/my-project-path/node_modules/vue-loader-v16/dist/templateLoader.js:32:37)

 @ ./src/App.vue?vue&type=template&id=7ba5bd90 1:0-319 1:0-319
 @ ./src/App.vue
 @ ./src/main.js
 @ multi (webpack)-dev-server/client?http://192.168.0.126:8080&sockPath=/sockjs-node (webpack)/hot/dev-server.js ./src/main.js

With Vite:

  TypeError: Cannot read property '2' of undefined
      at injectProp (/my-project-path/node_modules/@vue/compiler-core/dist/compiler-core.cjs.js:443:82)
      at createChildrenCodegenNode (/my-project-path/node_modules/@vue/compiler-core/dist/compiler-core.cjs.js:2841:9)
      at createCodegenNodeForBranch (/my-project-path/node_modules/@vue/compiler-core/dist/compiler-core.cjs.js:2797:62)
      at Array.<anonymous> (/my-project-path/node_modules/@vue/compiler-core/dist/compiler-core.cjs.js:2696:38)
      at traverseNode (/my-project-path/node_modules/@vue/compiler-core/dist/compiler-core.cjs.js:1681:19)
      at traverseChildren (/my-project-path/node_modules/@vue/compiler-core/dist/compiler-core.cjs.js:1624:9)
      at traverseNode (/my-project-path/node_modules/@vue/compiler-core/dist/compiler-core.cjs.js:1675:13)
      at traverseChildren (/my-project-path/node_modules/@vue/compiler-core/dist/compiler-core.cjs.js:1624:9)
      at traverseNode (/my-project-path/node_modules/@vue/compiler-core/dist/compiler-core.cjs.js:1675:13)
      at transform (/my-project-path/node_modules/@vue/compiler-core/dist/compiler-core.cjs.js:1566:5)

@yyx990803 The problem reappears when adding a new node with v-else, regardless of whether v-once is present in the new node. I updated my example: https://github.com/luka-mikec/vue-once-if-issue-demo/blob/master/src/App.vue

Edit: there is a problem with v-once inside v-for too, I added an example to the same file.

@yyx990803 the problem is present in v3.0 too

It's indeed failing on the template explorer