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// edition:2018
trait Trait<'a, 'b> {}
impl<T> Trait<'_, '_> for T {}
// `Ordinary<'a> <: Ordinary<'b>` if `'a: 'b`, as with most types.
//
// I am purposefully avoiding the terms co- and contra-variant because
// their application to regions depends on how you interpreted Rust
// regions. -nikomatsakis
struct Ordinary<'a>(&'a u8);
// Here we need something outlived by `'a` *and* outlived by `'b`, but
// we can only name `'a` and `'b` (and neither suits). So we get an
// error. Somewhat unfortunate, though, since the caller would have to
// consider the loans for both `'a` and `'b` alive.
fn upper_bounds<'a, 'b>(a: Ordinary<'a>, b: Ordinary<'b>) -> impl Trait<'a, 'b>
{
// We return a value:
//
// ```
// 'a: '0
// 'b: '1
// '0 in ['a, 'b]
// ```
//
// but we don't have it.
//
// We are forced to pick that '0 = 'e, because only 'e is outlived by *both* 'a and 'b.
if condition() { a } else { b }
//~^ ERROR hidden type for `impl Trait<'a, 'b>` captures lifetime that does not appear in bounds
}
fn condition() -> bool {
true
}
fn main() {}
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