Why non_lifetime_binders trait cant work with auto trait

language design
1

I have tried this code :

#![feature(specialization)]
#![feature(non_lifetime_binders)]
#![feature(auto_traits)]
#![feature(negative_impls)]

auto trait Default {}
pub trait Bar<T:?Sized>{}
pub trait Foo:for<T> Bar<T>{}


pub struct Impl;

// usize is a non default implementation, do some special work for Bar
impl ! Default for usize {}
impl Bar<usize> for Impl {}

// for default implementation ,do some general work for Bar
impl<T:?Sized> Bar<T> for Impl where T: Default {}

// we have done implemented for any Type T Bar<T> for for `Impl`, try to implement `Foo` for `Impl`. but error occurred.
impl Foo for Impl{}

fn main(){}

what i expect is:compile pass.

but it shows error:

error[E0277]: the trait bound `T: Default` is not satisfied
  --> src/main.rs:72:14
   |
72 | impl Foo for Impl{}
   |              ^^^^ the trait `Default` is not implemented for `T`
   |
   = help: the following other types implement trait `Bar<T>`:
             <Impl as Bar<T>>
             <Impl as Bar<usize>>
note: required for `Impl` to implement `Bar<T>`
  --> src/main.rs:69:16
   |
69 | impl<T:?Sized> Bar<T> for Impl where T: Default {}
   |                ^^^^^^     ^^^^          ------- unsatisfied trait bound introduced here
note: required by a bound in `Foo`
  --> src/main.rs:59:15
   |
59 | pub trait Foo:for<T> Bar<T>{}
   |               ^^^^^^^^^^^^^ required by this bound in `Foo`

It's seemed that rust compiler deduce whether the Impl satisfy for<T> Bar<T> by just searching for impl<T:?Sized> Bar<T> for Impl instead of really judging for any type T whether Bar<T> is implemented for Impl.

So, my question is :

  • Is it expected or a bug ?
  • If it is expected, any reason for not to compile ?
2

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