Rust expects two levels of boxing for generator while I only specified one

I am encountering a compiler error for something that I feel should work.

I tried this code (note generators are nightly-only at the time of writing):

#![feature(generators, generator_trait)]use std::ops::Generator;struct Value {}struct Container<G: Generator<Yield = Value, Return = ()>> {    generator: Box<G>,}impl Container<Box<Generator<Yield = Value, Return = ()>>> {    pub fn new(&mut self) -> Box<Self> {        let generator: Box<Generator<Yield = Value, Return = ()>> = Box::new(|| loop {            yield Value {}        });        Box::new(Container {            generator: generator,        })    }}fn main() {}

where I get this error:

error[E0308]: mismatched types  --> src/main.rs:20:24   |20 |             generator: generator,   |                        ^^^^^^^^^ expected struct `std::boxed::Box`, found trait std::ops::Generator   |   = note: expected type `std::boxed::Box<std::boxed::Box<std::ops::Generator<Yield=Value, Return=()>>>`              found type `std::boxed::Box<std::ops::Generator<Yield=Value, Return=()>>`error: aborting due to previous error

I don't understand why two levels of boxing are expected here, I only asked for one (Box<G>).

It looks like Generator is indeed a trait, not an alias for Box<...>. I can't think of other explanations.

I can easily resolve the error by replacing Box<G> by G, but I want to know why my way does not work (could it be a bug?).

Nightly version 1.28.0-nightly (2018-06-15 967c1f3be1c9ce0469ae) in debug mode on the playground (but I have a similar issue locally with more complex code).