src/test/ui/underscore-lifetime/where-clause-inherent-impl-underscore.rs

// revisions: rust2015 rust2018
//[rust2018] edition:2018

trait WithType<T> {}
trait WithRegion<'a> { }

struct Foo<T> {
    t: T
}

impl<T> Foo<T>
where
    T: WithRegion<'_>
//[rust2015,rust2018]~^ ERROR `'_` cannot be used here
{ }

fn main() {}