rust/src/test/ui/impl-trait/multiple-lifetimes/ordinary-bounds-pick-original.rs

// edition:2018
// build-pass (FIXME(62277): could be check-pass?)
// revisions: migrate mir
//[mir]compile-flags: -Z borrowck=mir

#![feature(member_constraints)]

trait Trait<'a, 'b> { }
impl<T> Trait<'_, '_> for T { }

// Here we wind up selecting `'a` and `'b` in the hidden type because
// those are the types that appear in the original values.

fn upper_bounds<'a, 'b>(a: &'a u8, b: &'b u8) -> impl Trait<'a, 'b> {
    // In this simple case, you have a hidden type `(&'0 u8, &'1 u8)` and constraints like
    //
    // ```
    // 'a: '0
    // 'b: '1
    // '0 in ['a, 'b]
    // '1 in ['a, 'b]
    // ```
    //
    // We use the fact that `'a: 0'` must hold (combined with the in
    // constraint) to determine that `'0 = 'a` must be the answer.
    (a, b)
}

fn main() { }