This repo takes a default vue 2.x app with vuex and follows the steps outlined in the @vue/compat upgrade workflow instructions. This repo shows that when started, the following will appear in the console:
vue.runtime.esm-bundler.js?1786:1404 Uncaught TypeError: Cannot read property 'state' of undefined
at Proxy.mappedState (vuex.esm.js?2f62:945)
at ComputedRefImpl.reactiveEffect [as effect] (vue.runtime.esm-bundler.js?1786:343)
at ComputedRefImpl.get value [as value] (vue.runtime.esm-bundler.js?1786:1165)
at Object.get [as counter] (vue.runtime.esm-bundler.js?1786:4515)
at Object.get (vue.runtime.esm-bundler.js?1786:9487)
at Proxy.eval (HelloWorld.vue?fdab:3)
at renderComponentRoot (vue.runtime.esm-bundler.js?1786:2550)
at componentEffect (vue.runtime.esm-bundler.js?1786:7280)
at reactiveEffect (vue.runtime.esm-bundler.js?1786:343)
at effect (vue.runtime.esm-bundler.js?1786:318)
yarn install
yarn serve
yarn build