mirror of https://github.com/rust-lang/rust
36 lines
1.2 KiB
Rust
36 lines
1.2 KiB
Rust
// 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
|
|
}
|