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// 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" equality 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` -- error.
// - `fn(fn(&?a u32)) == for<'b> fn(fn(&'b u32))` :-
// - `fn(&?b u32) == fn(&?a u32)`
// - `&?a u32 == &?b u32`
// - `?a == ?b` -- OK.
// - So the unification fails.
foo::<()>();
//~^ ERROR implementation of `Trait` is not general enough
}
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