Cf https://github.com/sbt/zinc/blob/1.0/internal/compiler-interface/src/main/contraband/type.json

Structure is not a subtype of SimpleType but Projection#prefix has to be a simple type this means that sbt cannot represent something like (Foo { type T })#T (the prefix is replaced by the empty type in simpleType).

I think we should just get rid of the SimpleType/Type separation, it's not useful.