Updated tests for issue rust-lang#65673.

Author: alexreg

src/test/ui/issues/issue-65673-fail.rs

@@ -0,0 +1,22 @@
+#![feature(trait_alias)]
+
+use std::any::Any;
+
+trait Trait {}
+
+trait WithType {
+    type Ctx: ?Sized;
+}
+
+trait Alias<T> = Any where T: Trait;
+
+// Note, we do not impl `Trait` for `()` here, so should expect the error below.
+
+impl<T> WithType for T {
+    type Ctx = dyn Alias<T>;
+}
+
+fn main() {
+    let _: Box<<() as WithType>::Ctx> = Box::new(());
+    //~^ ERROR the trait bound `(): Trait` is not satisfied [E0277]
+}

src/test/ui/issues/issue-65673-fail.stderr

@@ -0,0 +1,11 @@
+error[E0277]: the trait bound `(): Trait` is not satisfied
+  --> $DIR/issue-65673-fail.rs:20:41
+   |
+LL |     let _: Box<<() as WithType>::Ctx> = Box::new(());
+   |                                         ^^^^^^^^^^^^ the trait `Trait` is not implemented for `()`
+   |
+   = note: required for the cast to the object type `dyn Alias<()>`
+
+error: aborting due to previous error
+
+For more information about this error, try `rustc --explain E0277`.

src/test/ui/issues/issue-65673.rs

@@ -1,12 +1,23 @@
-#![feature(trait_alias)] // Enabled to reduce stderr output, but can be triggered even if disabled.
+// run-pass
+
+#![feature(trait_alias)]
+
+use std::any::Any;
+
 trait Trait {}
+
 trait WithType {
-    type Ctx;
+    type Ctx: ?Sized;
 }
-trait Alias<T> = where T: Trait;
+
+trait Alias<T> = Any where T: Trait;
+
+impl Trait for () {}
 
 impl<T> WithType for T {
     type Ctx = dyn Alias<T>;
-//~^ ERROR the size for values of type `(dyn Trait + 'static)` cannot be known at compilation time
 }
-fn main() {}
+
+fn main() {
+    let _: Box<<() as WithType>::Ctx> = Box::new(());
+}