| // Test a case where variance and higher-ranked types interact in surprising ways. |
| // |
| // In particular, we test this pattern in trait solving, where it is not connected |
| // to any part of the source code. |
| |
| trait Trait<T> {} |
| |
| fn foo<T>() |
| where |
| T: Trait<for<'b> fn(fn(&'b u32))>, |
| { |
| } |
| |
| impl<'a> Trait<fn(fn(&'a u32))> for () {} |
| |
| fn main() { |
| // Here, proving that `(): Trait<for<'b> fn(&'b u32)>` uses the impl: |
| // |
| // - The impl provides the clause `forall<'a> { (): Trait<fn(fn(&'a u32))> }` |
| // - We instantiate `'a` existentially to get `(): Trait<fn(fn(&?a u32))>` |
| // - We unify `fn(fn(&?a u32))` with `for<'b> fn(fn(&'b u32))` -- this does a |
| // "bidirectional" subtyping check, so we wind up with: |
| // - `fn(fn(&?a u32)) <: for<'b> fn(fn(&'b u32))` :- |
| // - `fn(&!b u32) <: fn(&?a u32)` |
| // - `&?a u32 <: &!b u32` |
| // - `?a: !'b` -- solveable if `?a` is inferred to `'static` |
| // - `for<'b> fn(fn(&'b u32)) <: fn(fn(&?a u32))` :- |
| // - `fn(&?a u32) <: fn(&?b u32)` |
| // - `&?b u32 <: &?a u32` |
| // - `?b: ?a` -- solveable if `?b` is inferred to `'static` |
| // - So the subtyping check succeeds, somewhat surprisingly. |
| // This is because we can use `'static`. |
| // |
| // NB. *However*, the reinstated leak-check gives an error here. |
| |
| foo::<()>(); |
| //~^ ERROR not satisfied |
| } |