? does not coerce Box<dyn Error + Send + Sync + 'static> to anyhow::Error

[That3Percent]

It is common in crates that take the anyhow approach of loosely typed dynamic error types to return Result<T, Box<dyn Error + Send + Sync + 'static>>.

Like anyhow, this type is convenient to use - automatically coercing from impl Error and even String.

But, when trying to interop with this common return value using ? the compiler stumbles with:

the trait std::marker::Sized is not implemented for dyn std::error::Error + std::marker::Send + std::marker::Sync

Here is a minimum reproducible case:

use anyhow;

fn dyn_error() -> Result<(), Box<dyn std::error::Error + Send + Sync + 'static>> {
    Ok(())
}

fn anyhow_error() -> Result<(), anyhow::Error> {
    dyn_error()?;
    Ok(())
}

[dtolnay]

I don't think this is something that could be fixed in anyhow. There is a missing impl of std::error::Error for Box<dyn Error> in std -- rust-lang/rust#60759. Once specialization is mature enough that the missing impl can be provided in std, it will fix this issue.