supertraits do not resolve correctly for dynamic dispatch trait bounds

[drahnr]

I tried this code:

https://play.rust-lang.org/?version=stable&mode=debug&edition=2018&gist=7e579f79329f28e5441b0291d22d60f9

where X is the supertrait in question.

use std::fmt;

pub trait X: 'static + fmt::Debug + fmt::Display + Send + Sync {}

impl<T: 'static + fmt::Debug + fmt::Display + Send + Sync> X for T {}


#[derive(Debug)]
enum Skippy {
    V(Box<dyn X>),
    Alpha,
    Beta,
}

impl fmt::Display for Skippy {
 	fn fmt(&self, fmt: &mut fmt::Formatter) -> fmt::Result {
		writeln!(fmt, "yak")
	}
}


// commenting out either of them makes is fail to compile

// impl From<Box<dyn X + Send + Sync>> for Skippy {
// 	fn from(e: Box<dyn X + Send + Sync>) -> Self {
// 		Self::V(e)
// 	}
// }


impl From<Box<dyn X>> for Skippy {
	fn from(e: Box<dyn X>) -> Self {
 		Self::V(e)
	}
}


#[inline(never)]
fn alpha() -> Box<dyn X> {
    Box::new(Skippy::Alpha)
}


#[inline(never)]
fn beta() -> Box<dyn X + Send + Sync + 'static> {
    Box::new(Skippy::Beta)
}

fn main() {
    Skippy::from(alpha());
    Skippy::from(beta());
}

I expected to see this happen: explanation

Work fine with just one From<Box<dyn X>> impl, since the traits are already included in the super trait.

Instead, this happened: explanation

Requirement to impl for multiple summed trait bounds.

Meta

rustc --version --verbose:

1.47.0

[drahnr]

CC @bkchr

[drahnr]

Still present in 1.49.0

[jplatte]

More generally, dyn X and dyn X + Send are apparently considered entirely separate types, even though X: Send. Here's a smaller example that shows this (playground):

pub trait Tr: Send {}

impl dyn Tr {
    fn foo() {
        println!("impl a");
    }
}

impl dyn Tr + Send {
    fn foo() {
        println!("impl b");
    }
}

fn main() {
    <dyn Tr>::foo();
    <dyn Tr + Send>::foo();
}

Which compiles successfully and prints

impl a
impl b

[jplatte]

CC @nikomatsakis who is probably the most knowledgable person about dyn Trait and all its problems.

[jplatte]

My above example works all the way back in 1.0.0 (if you remove the dyn).

[Xanewok]

I'd be happy to tackle this on. @nikomatsakis could you give some pointers on why things work the way they do and where should I look/how do you imagine implementing that? 🙏