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diff --git a/tests/ui/generics/issue-61631-default-type-param-can-reference-self-in-trait.rs b/tests/ui/generics/issue-61631-default-type-param-can-reference-self-in-trait.rs
new file mode 100644
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+++ b/tests/ui/generics/issue-61631-default-type-param-can-reference-self-in-trait.rs
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+#![crate_type="lib"]
+
+// rust-lang/rust#61631: The use of `Self` in the defaults of generic
+// types in a *trait* definition are allowed.
+//
+// It *must* be accepted; we have used this pattern extensively since
+// Rust 1.0 (see e.g. `trait Add<Rhs=Self>`).
+trait Tnobound<P = Self> {}
+
+impl Tnobound for () { }
+
+// This variant is accepted at the definition site; but it will be
+// rejected at every possible usage site (such as the one immediately
+// below). Maybe one day we will attempt to catch it at the definition
+// site, but today this is accepted due to compiler implementation
+// limitations.
+trait Tsized<P: Sized = [Self]> {}
+
+impl Tsized for () {}
+//~^ ERROR the size for values of type `[()]` cannot be known at compilation time [E0277]