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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(&'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
}
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