Doesn't compile if a decoder for a recursive structure is not defined at top level

[malidukica]

I wasn't sure if this should be reported in elm/compiler or here, but it is a curious case, and I lost some hours trying to figure out why it won't work and rewriting it until I moved the decoder to top level.

A contrived example can be found at: https://ellie-app.com/bjMK9h2bvDga1