The language, in general, wants you to be explicit about what you're working with. So implicit access to interface members isn't a thing, you need to say that "x is this interface type" or cast.

In F# 6 things are changing a bit, and may possibly change in the future. There are various classes of cases where the type information is known, but the compiler still required you to explicitly cast things to be happy. That's no longer case. This link[0] has 3 examples that don't compile with F# 5 but will compile with F# 6.

[0] https://sharplab.io/#v2:DYLgZgzgNALiBOBXAdgHwPToAQFMAeAhgLYA...