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#![feature(type_alias_impl_trait)]
use std::fmt::Debug;
type Foo = impl Debug;
fn is_send<T: Send>() {}
fn not_good() {
// This function does not define `Foo`,
// so it can actually check auto traits on the hidden type without
// risking cycle errors.
is_send::<Foo>();
}
#[define_opaque(Foo)]
fn not_gooder() -> Foo {
// Constrain `Foo = u32`
let x: Foo = 22_u32;
// while we could know this from the hidden type, it would
// need extra roundabout logic to support it.
is_send::<Foo>();
//~^ ERROR: type annotations needed: cannot satisfy `Foo: Send`
x
}
fn main() {}
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