--- /dev/null
+// check-pass
+
+trait Mirror {
+ type Other;
+}
+
+#[derive(Debug)]
+struct Even(usize);
+struct Odd;
+
+impl Mirror for Even {
+ type Other = Odd;
+}
+
+impl Mirror for Odd {
+ type Other = Even;
+}
+
+trait Dyn<T: Mirror>: AsRef<<T as Mirror>::Other> {}
+
+impl Dyn<Odd> for Even {}
+
+impl AsRef<Even> for Even {
+ fn as_ref(&self) -> &Even {
+ self
+ }
+}
+
+fn code<T: Mirror>(d: &dyn Dyn<T>) -> &T::Other {
+ d.as_ref()
+}
+
+fn main() {
+ println!("{:?}", code(&Even(22)));
+}
--- /dev/null
+fn t7p<A, B, C>(f: impl Fn(B) -> C, g: impl Fn(A) -> B) -> impl Fn(A) -> C {
+ move |a: A| -> C { f(g(a)) }
+}
+
+fn t8n<A, B, C>(f: impl Fn(A) -> B, g: impl Fn(A) -> C) -> impl Fn(A) -> (B, C)
+where
+ A: Copy,
+{
+ move |a: A| -> (B, C) {
+ let b = a;
+ let fa = f(a);
+ let ga = g(b);
+ (fa, ga)
+ }
+}
+
+fn main() {
+ let f = |(_, _)| {};
+ let g = |(a, _)| a;
+ let t7 = |env| |a| |b| t7p(f, g)(((env, a), b));
+ let t8 = t8n(t7, t7p(f, g));
+ //~^ ERROR: expected a `Fn<(_,)>` closure, found `impl Fn<(((_, _), _),)>
+}
--- /dev/null
+error[E0277]: expected a `Fn<(_,)>` closure, found `impl Fn<(((_, _), _),)>`
+ --> $DIR/issue-59494.rs:21:22
+ |
+LL | fn t8n<A, B, C>(f: impl Fn(A) -> B, g: impl Fn(A) -> C) -> impl Fn(A) -> (B, C)
+ | ---------- required by this bound in `t8n`
+...
+LL | let t8 = t8n(t7, t7p(f, g));
+ | ^^^^^^^^^ expected an `Fn<(_,)>` closure, found `impl Fn<(((_, _), _),)>`
+ |
+ = help: the trait `Fn<(_,)>` is not implemented for `impl Fn<(((_, _), _),)>`
+
+error: aborting due to previous error
+
+For more information about this error, try `rustc --explain E0277`.
--- /dev/null
+// check-pass
+
+pub trait Trait1 {
+ type C;
+}
+
+struct T1;
+impl Trait1 for T1 {
+ type C = usize;
+}
+pub trait Callback<T: Trait1>: FnMut(<T as Trait1>::C) {}
+impl<T: Trait1, F: FnMut(<T as Trait1>::C)> Callback<T> for F {}
+
+pub struct State<T: Trait1> {
+ callback: Option<Box<dyn Callback<T>>>,
+}
+impl<T: Trait1> State<T> {
+ fn new() -> Self {
+ Self { callback: None }
+ }
+ fn test_cb(&mut self, d: <T as Trait1>::C) {
+ (self.callback.as_mut().unwrap())(d)
+ }
+}
+
+fn main() {
+ let mut s = State::<T1>::new();
+ s.test_cb(1);
+}
--- /dev/null
+// check-pass
+
+fn any<T>() -> T {
+ loop {}
+}
+
+trait Foo {
+ type V;
+}
+
+trait Callback<T: Foo>: Fn(&T, &T::V) {}
+impl<T: Foo, F: Fn(&T, &T::V)> Callback<T> for F {}
+
+struct Bar<T: Foo> {
+ callback: Box<dyn Callback<T>>,
+}
+
+impl<T: Foo> Bar<T> {
+ fn event(&self) {
+ (self.callback)(any(), any());
+ }
+}
+
+struct A;
+struct B;
+impl Foo for A {
+ type V = B;
+}
+
+fn main() {
+ let foo = Bar::<A> { callback: Box::new(|_: &A, _: &B| ()) };
+ foo.event();
+}