| // 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(&'b u32)>, |
| { |
| } |
| |
| impl<'a> Trait<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(&'a u32)> }` |
| // - We instantiate `'a` existentially to get `(): Trait<fn(&?a u32)>` |
| // - We unify `fn(&?a u32)` with `for<'b> fn(&'b u32)` -- this does a |
| // "bidirectional" subtyping check, so we wind up with: |
| // - `fn(&?a u32) <: for<'b> fn(&'b u32)` :- |
| // - `&'!b u32 <: &?a u32` |
| // - `!'b: ?a` -- solveable if `?a` is inferred to `'empty` |
| // - `for<'b> fn(&'b u32) <: fn(&?a u32)` :- |
| // - `&?a u32 u32 <: &?b u32` |
| // - `?a: ?b` -- solveable if `?b` is also inferred to `'empty` |
| // - So the subtyping check succeeds, somewhat surprisingly. |
| // This is because we can use `'empty`. |
| // |
| // NB. *However*, the reinstated leak-check gives an error here. |
| |
| foo::<()>(); |
| //~^ ERROR implementation of `Trait` is not general enough |
| } |