Vue: Vue.extend causes nested mixins to be merged twice

[KaelWD]

Version

2.5.17

Reproduction link

https://codepen.io/anon/pen/ReNVra?editors=0011

Steps to reproduce

Open the console

What is expected?

"mixinA created"
"mixinB created"
"ChildComponent created"

What is actually happening?

"mixinA created"
"mixinA created"
"mixinB created"
"ChildComponent created"

---

MixinB.options still has mixins, which gets applied again if used in mixins: [] or extends: even though Vue.extend() has already done that

[posva]

Don't use Vue.extend on a mixin, a mixins are plain objects

[KaelWD]

Not if you use typescript they aren't.

[johnleider]

Can you provide an example of how to handle this when using typescript @posva ?