error: ambiguous lifetime bound in `impl Trait`
  --> $DIR/feature-gate-member-constraints.rs:4:43
   |
LL | fn foo<'a, 'b>(x: &'a u32, y: &'b u32) -> impl Trait<'a, 'b> {
   |                                           ^^^^^^^^^^^^^^^^^^ neither `'a` nor `'b` outlives the other
   |
   = help: add #![feature(member_constraints)] to the crate attributes to enable

error: aborting due to previous error