Traits are resolved incorrectly when dyn trait is used in a struct.

[wdanilo]

Consider the following code:

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

use core::any::TypeId;

auto trait NotSame {}
impl<T> ! NotSame for (T, T) {}


#[derive(Debug)]
pub struct EventData {
    pub id:  TypeId,
    payload: Box<dyn std::any::Any>,
}

impl EventData {
    pub fn new<T: 'static>(payload: T) -> Self {
        let id = TypeId::of::<T>();
        let payload = Box::new(payload);
        Self { id, payload }
    }
}

impl<T: 'static> From<T> for EventData
where (T, EventData): NotSame
{
    fn from(t: T) -> Self {
        Self::new(t)
    }
}

fn main() {
    let v: EventData = ().into();
}

It does not compile:

error[[E0277]](https://doc.rust-lang.org/nightly/error-index.html#E0277): the trait bound `(dyn Any + 'static): NotSame` is not satisfied in `((), EventData)`
  --> src/main.rs:34:24
   |
34 |     let v: EventData = ().into();
   |                        ^^ ---- required by a bound introduced by this call
   |                        |
   |                        within `((), EventData)`, the trait `NotSame` is not implemented for `(dyn Any + 'static)`
   |
   = note: required because it appears within the type `PhantomData<(dyn Any + 'static)>`
   = note: required because it appears within the type `Unique<(dyn Any + 'static)>`
   = note: required because it appears within the type `Box<(dyn Any + 'static)>`
note: required because it appears within the type `EventData`

However, if we remove the field containing std::any::Any, it compiles fine:

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

use core::any::TypeId;

auto trait NotSame {}
impl<T> ! NotSame for (T, T) {}


#[derive(Debug)]
pub struct EventData {
}

impl EventData {
    pub fn new<T: 'static>(payload: T) -> Self {
        Self {}
    }
}

impl<T: 'static> From<T> for EventData
where (T, EventData): NotSame
{
    fn from(t: T) -> Self {
        Self::new(t)
    }
}

fn main() {
    let v: EventData = ().into();
}

Another strange thing is that if we change ().into() to EventData::from(()), then none of these codes compiles.

Please note that no types were changed. Click here to see live demo

[wdanilo]

Edit: this does not work with any dyn trait, even custom one.

[BGR360]

I've hit this before when trying to do the exact same NotSame thing you're trying to do. It's a quirk with the way auto traits work. Basically auto traits don't automatically apply to unsized types or something. For me I ended up needing to do this: https://github.com/BGR360/tres/blob/4db4af13162396bbf1a9bed10d3659de74838678/src/traced.rs#L262-L266

@rustbot label +T-compiler +requires-nightly +F-auto_traits

[wdanilo]

@BGR360 hmmm, but my example does not use any unsized arguments here. The struct does include dyn trait. See the example I linked in my post. It does not compile even if I add + Sized there.