Add test for issue #74824

Author: aDotInTheVoid

src/test/ui/generic-associated-types/issue-74824.rs

@@ -0,0 +1,27 @@
+#![feature(generic_associated_types)]
+#![feature(associated_type_defaults)]
+#![allow(incomplete_features)]
+
+use std::ops::Deref;
+
+trait UnsafeCopy {
+    type Copy<T>: Copy = Box<T>;
+    //~^ ERROR the trait bound `Box<T>: Copy` is not satisfied
+    //~^^ ERROR the trait bound `T: Clone` is not satisfied
+    fn copy<T>(x: &Self::Copy<T>) -> Self::Copy<T> {
+        *x
+    }
+}
+
+impl<T> UnsafeCopy for T {}
+
+fn main() {
+    let b = Box::new(42usize);
+    let copy = <()>::copy(&b);
+
+    let raw_b = Box::deref(&b) as *const _;
+    let raw_copy = Box::deref(&copy) as *const _;
+
+    // assert the addresses.
+    assert_eq!(raw_b, raw_copy);
+}

src/test/ui/generic-associated-types/issue-74824.stderr

@@ -0,0 +1,27 @@
+error[E0277]: the trait bound `Box<T>: Copy` is not satisfied
+  --> $DIR/issue-74824.rs:8:5
+   |
+LL |     type Copy<T>: Copy = Box<T>;
+   |     ^^^^^^^^^^^^^^----^^^^^^^^^^
+   |     |             |
+   |     |             required by this bound in `UnsafeCopy::Copy`
+   |     the trait `Copy` is not implemented for `Box<T>`
+
+error[E0277]: the trait bound `T: Clone` is not satisfied
+  --> $DIR/issue-74824.rs:8:5
+   |
+LL |     type Copy<T>: Copy = Box<T>;
+   |     ^^^^^^^^^^^^^^----^^^^^^^^^^
+   |     |             |
+   |     |             required by this bound in `UnsafeCopy::Copy`
+   |     the trait `Clone` is not implemented for `T`
+   |
+   = note: required because of the requirements on the impl of `Clone` for `Box<T>`
+help: consider restricting type parameter `T`
+   |
+LL |     type Copy<T: Clone>: Copy = Box<T>;
+   |                ^^^^^^^
+
+error: aborting due to 2 previous errors
+
+For more information about this error, try `rustc --explain E0277`.