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diff --git a/src/test/ui/traits/wf-object/no-duplicates.rs b/src/test/ui/traits/wf-object/no-duplicates.rs
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+++ b/src/test/ui/traits/wf-object/no-duplicates.rs
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+// The purpose of this test is to demonstrate that duplicating object safe traits
+// that are not auto-traits is rejected even though one could reasonably accept this.
+
+// Some arbitrary object-safe trait:
+trait Obj {}
+
+// Demonstrate that recursive expansion of trait aliases doesn't affect stable behavior:
+type _0 = dyn Obj + Obj;
+//~^ ERROR only auto traits can be used as additional traits in a trait object [E0225]
+
+// Some variations:
+
+type _1 = dyn Send + Obj + Obj;
+//~^ ERROR only auto traits can be used as additional traits in a trait object [E0225]
+
+type _2 = dyn Obj + Send + Obj;
+//~^ ERROR only auto traits can be used as additional traits in a trait object [E0225]
+
+type _3 = dyn Obj + Send + Send; // But it is OK to duplicate auto traits.
+
+// Take higher ranked types into account.
+
+// Note that `'a` and `'b` are intentionally different to make sure we consider
+// them semantically the same.
+trait ObjL<'l> {}
+type _4 = dyn for<'a> ObjL<'a> + for<'b> ObjL<'b>;
+//~^ ERROR only auto traits can be used as additional traits in a trait object [E0225]
+
+trait ObjT<T> {}
+type _5 = dyn ObjT<for<'a> fn(&'a u8)> + ObjT<for<'b> fn(&'b u8)>;
+//~^ ERROR only auto traits can be used as additional traits in a trait object [E0225]
+
+fn main() {}