rust/tests/ui/higher-rank-trait-bounds/hrtb-exists-forall-trait-contravariant.rs

// 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
}