error[E0621]: explicit lifetime required in the type of `y`
  --> ../../ex1-return-one-existing-name-if-else.rs:12:8
   |
11 | fn foo<'a>(t: bool, x: &'a i32, y: &i32) -> &'a i32 {
   |                                 - consider changing the type of `y` to `&'a i32`
12 |     if t { x } else { y } //~ ERROR explicit lifetime
   |        ^ lifetime `'a` required