add known-bug test for unsound issue 57893

tests/ui/coherence/indirect-impl-for-trait-obj-coherence.rs

@@ -0,0 +1,25 @@
+// check-pass
+// known-bug: #57893
+
+// Should fail. Because we see an impl that uses a certain associated type, we
+// type-check assuming that impl is used. However, this conflicts with the
+// "implicit impl" that we get for trait objects, violating coherence.
+
+trait Object<U> {
+    type Output;
+}
+
+impl<T: ?Sized, U> Object<U> for T {
+    type Output = U;
+}
+
+fn foo<T: ?Sized, U>(x: <T as Object<U>>::Output) -> U {
+    x
+}
+
+#[allow(dead_code)]
+fn transmute<T, U>(x: T) -> U {
+    foo::<dyn Object<U, Output = T>, U>(x)
+}
+
+fn main() {}