An inherent impl was written on a dyn auto trait.
Erroneous code example:
#![feature(auto_traits)]
auto trait AutoTrait {}
impl dyn AutoTrait {}Dyn objects allow any number of auto traits, plus at most one non-auto trait. The non-auto trait becomes the “principal trait”.
When checking if an impl on a dyn trait is coherent, the principal trait is normally the only one considered. Since the erroneous code has no principal trait, it cannot be implemented at all.
Working example:
#![feature(auto_traits)]
trait PrincipalTrait {}
auto trait AutoTrait {}
impl dyn PrincipalTrait + AutoTrait + Send {}