| //@ edition:2018 |
| //@ check-pass |
| |
| trait Trait<'a, 'b> {} |
| impl<T> Trait<'_, '_> for T {} |
| |
| // `Invert<'a> <: Invert<'b>` if `'b: 'a`, unlike 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 Invert<'a>(fn(&'a u8)); |
| |
| fn upper_bounds<'a, 'b, 'c, 'd, 'e>(a: Invert<'a>, b: Invert<'b>) -> impl Trait<'d, 'e> |
| where |
| 'c: 'a, |
| 'c: 'b, |
| 'd: 'c, |
| { |
| // Representing the where clauses as a graph, where `A: B` is an |
| // edge `B -> A`: |
| // |
| // ``` |
| // 'a -> 'c -> 'd |
| // ^ |
| // | |
| // 'b |
| // ``` |
| // |
| // Meanwhile we return a value &'0 u8 where we have the constraints: |
| // |
| // ``` |
| // '0: 'a |
| // '0: 'b |
| // '0 in ['d, 'e] |
| // ``` |
| // |
| // Here, ignoring the "in" constraint, the minimal choice for `'0` |
| // is `'c`, but that is not in the "in set". Still, that reduces |
| // the range of options in the "in set" to just `'d` (`'e: 'c` |
| // does not hold). |
| let p = if condition() { a } else { b }; |
| p |
| } |
| |
| fn condition() -> bool { |
| true |
| } |
| |
| fn main() {} |
