1 // Test a case where variance and higher-ranked types interact in surprising ways.
3 // In particular, we test this pattern in trait solving, where it is not connected
4 // to any part of the source code.
10 T: Trait<for<'b> fn(&'b u32)>,
14 impl<'a> Trait<fn(&'a u32)> for () {}
17 // Here, proving that `(): Trait<for<'b> fn(&'b u32)>` uses the impl:
19 // - The impl provides the clause `forall<'a> { (): Trait<fn(&'a u32)> }`
20 // - We instantiate `'a` existentially to get `(): Trait<fn(&?a u32)>`
21 // - We unify `fn(&?a u32)` with `for<'b> fn(&'b u32)` -- this does a
22 // "bidirectional" subtyping check, so we wind up with:
23 // - `fn(&?a u32) <: for<'b> fn(&'b u32)` :-
24 // - `&'!b u32 <: &?a u32`
25 // - `!'b: ?a` -- solveable if `?a` is inferred to `'empty`
26 // - `for<'b> fn(&'b u32) <: fn(&?a u32)` :-
27 // - `&?a u32 u32 <: &?b u32`
28 // - `?a: ?b` -- solveable if `?b` is also inferred to `'empty`
29 // - So the subtyping check succeeds, somewhat surprisingly.
30 // This is because we can use `'empty`.
32 // NB. *However*, the reinstated leak-check gives an error here.
35 //~^ ERROR implementation of `Trait` is not general enough