error[E0404]: expected trait, found type alias `Foo`
  --> $DIR/issue-3907.rs:11:6
   |
LL | impl Foo for S {
   |      ^^^ type aliases cannot be used as traits
   |
help: you might have meant to use `#![feature(trait_alias)]` instead of a `type` alias
   |
LL - type Foo = dyn issue_3907::Foo;
LL + trait Foo = dyn issue_3907::Foo;
   |
help: consider importing this trait instead
   |
LL + use issue_3907::Foo;
   |

error: aborting due to 1 previous error

For more information about this error, try `rustc --explain E0404`.