Add some regression tests

Closes #56229
Closes #59494
Closes #70746
Closes #73229

src/test/ui/issues/issue-56229.rs

@@ -0,0 +1,35 @@
+// 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)));
+}

src/test/ui/issues/issue-59494.rs

@@ -0,0 +1,23 @@
+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<(((_, _), _),)>
+}

src/test/ui/issues/issue-59494.stderr

@@ -0,0 +1,14 @@
+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`.

src/test/ui/issues/issue-70746.rs

@@ -0,0 +1,29 @@
+// 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);
+}

src/test/ui/issues/issue-73229.rs

@@ -0,0 +1,33 @@
+// 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();
+}