Rust complains about trait assoc type not being WF when we assume a triat impl (which is inherently WF)

[RalfJung]

Consider this code:

trait Map<K, V> {}

trait Machine {
    type Tag;
    type MyMap: Map<usize, Self::Tag> + Default;
}

struct Context<M: Machine>(M::MyMap);

impl<M: Machine<Tag = ()>> Context<M> {
    fn new() -> Self {
        Context(Default::default())
    }
}

It fails to compile with

error[E0277]: the trait bound `<M as Machine>::MyMap: Map<usize, ()>` is not satisfied
  --> src/lib.rs:10:1
   |
10 | / impl<M: Machine<Tag = ()>> Context<M> {
11 | |     fn new() -> Self {
12 | |         Context(Default::default())
13 | |     }
14 | | }
   | |_^ the trait `Map<usize, ()>` is not implemented for `<M as Machine>::MyMap`
   |
   = help: consider adding a `where <M as Machine>::MyMap: Map<usize, ()>` bound
note: required by `Machine`
  --> src/lib.rs:3:1
   |
3  | trait Machine {
   | ^^^^^^^^^^^^^

error[E0277]: the trait bound `<M as Machine>::MyMap: Map<usize, ()>` is not satisfied
  --> src/lib.rs:11:5
   |
11 | /     fn new() -> Self {
12 | |         Context(Default::default())
13 | |     }
   | |_____^ the trait `Map<usize, ()>` is not implemented for `<M as Machine>::MyMap`
   |
   = help: consider adding a `where <M as Machine>::MyMap: Map<usize, ()>` bound
note: required by `Machine`
  --> src/lib.rs:3:1
   |
3  | trait Machine {
   | ^^^^^^^^^^^^^

error: aborting due to 2 previous errors

But that is strange, because the M: Machine impl must be well-formed, so of course that bound is met.

[rkarp]

This looks like another duplicate of #24159.

[RalfJung]

It does indeed, thanks! Closing this.