@nikomatsakis So, the outstanding issue we need to fix is multiple traits in a trait object that come in via (one or more) trait aliases. This should error, like it does right now when you just use
dyn TraitA + TraitB.
Basically, point 1 from my old PR https://github.com/rust-lang/rust/pull/55994 needs to be factored out and updated. There were possibly some complications though, you were mentioning?