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.
12 T: Trait<for<'b> fn(fn(&'b u32))>,
16 impl<'a> Trait<fn(fn(&'a u32))> for () {}
19 // Here, proving that `(): Trait<for<'b> fn(&'b u32)>` uses the impl:
21 // - The impl provides the clause `forall<'a> { (): Trait<fn(fn(&'a u32))> }`
22 // - We instantiate `'a` existentially to get `(): Trait<fn(fn(&?a u32))>`
23 // - We unify `fn(fn(&?a u32))` with `for<'b> fn(fn(&'b u32))` -- this does a
24 // "bidirectional" subtyping check, so we wind up with:
25 // - `fn(fn(&?a u32)) <: for<'b> fn(fn(&'b u32))` :-
26 // - `fn(&!b u32) <: fn(&?a u32)`
27 // - `&?a u32 <: &!b u32`
28 // - `?a: !'b` -- solveable if `?a` is inferred to `'static`
29 // - `for<'b> fn(fn(&'b u32)) <: fn(fn(&?a u32))` :-
30 // - `fn(&?a u32) <: fn(&?b u32)`
31 // - `&?b u32 <: &?a u32`
32 // - `?b: ?a` -- solveable if `?b` is inferred to `'static`
33 // - So the subtyping check succeeds, somewhat surprisingly.
34 // This is because we can use `'static`.