Opaque type subtyping with generics does not maintain exactness

Hello beloved type people,

I believe I'm experiencing an issue having to do with opaque types, generics and exactness. I'm afraid it's not demonstrable directly in flow.org/try/ as it requires operating outside the module where the opaque type is defined.

My use case is that I want to be able to flag values with types, so what I need is some kind of container type. I want to be able to create functions which expect values having been explicitly flagged by the developer, so the type needs to be opaque. I also want to be able to pass the flagged values to functions expecting the unflagged value. These taken together mean I need something of the form `opaque type F: A = A`.

This mostly works great. There's a problem if you assign an exact type to the type argument `A`, though: you can't assign a value of `F` to a variable of `A`, because `A` will be exact but `F` will not.

PS. Please do let me know if I can improve on this issue somehow, it's my first one here! 🙏