What problem does this feature solve?

there is app.provide('myVar', myVar) for the injection of variable.

but there is no the corresponding way to get the variable in plugin install.

( I found app._context.provides.myVar but not sure whether I can/should use it. )

What does the proposed API look like?

app.inject('myVar')

(it has been addressed by other developers in https://github.com/vuejs/composition-api/issues/288)

some problem