Regression tests.

Update: incorporate review feedback.

src/test/ui/generics/issue-61631-default-type-param-can-reference-self-in-trait.rs

@@ -0,0 +1,20 @@
+#![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]

src/test/ui/generics/issue-61631-default-type-param-can-reference-self-in-trait.stderr

@@ -0,0 +1,12 @@
+error[E0277]: the size for values of type `[()]` cannot be known at compilation time
+  --> $DIR/issue-61631-default-type-param-can-reference-self-in-trait.rs:19:6
+   |
+LL | impl Tsized for () {}
+   |      ^^^^^^ doesn't have a size known at compile-time
+   |
+   = help: the trait `std::marker::Sized` is not implemented for `[()]`
+   = note: to learn more, visit <https://doc.rust-lang.org/book/ch19-04-advanced-types.html#dynamically-sized-types-and-the-sized-trait>
+
+error: aborting due to previous error
+
+For more information about this error, try `rustc --explain E0277`.

src/test/ui/generics/issue-61631-default-type-param-cannot-reference-self.rs

@@ -0,0 +1,45 @@
+#![crate_type="lib"]
+
+// rust-lang/rust#61631: Uses of `Self` in the defaults of generic
+// types for ADT's are not allowed. We justify this because the `Self`
+// type could be considered the "final" type parameter, that is only
+// well-defined after all of the other type parameters on the ADT have
+// been instantiated.
+//
+// These were previously were ICE'ing at the usage point anyway (see
+// `demo_usages` below), so there should not be any backwards
+// compatibility concern.
+
+struct Snobound<'a, P = Self> { x: Option<&'a P> }
+//~^ ERROR type parameters cannot use `Self` in their defaults [E0735]
+
+enum Enobound<'a, P = Self> { A, B(Option<&'a P>) }
+//~^ ERROR type parameters cannot use `Self` in their defaults [E0735]
+
+union Unobound<'a, P = Self> { x: i32, y: Option<&'a P> }
+//~^ ERROR type parameters cannot use `Self` in their defaults [E0735]
+
+// Disallowing `Self` in defaults sidesteps need to check the bounds
+// on the defaults in cases like these.
+
+struct Ssized<'a, P: Sized = [Self]> { x: Option<&'a P> }
+//~^ ERROR type parameters cannot use `Self` in their defaults [E0735]
+
+enum Esized<'a, P: Sized = [Self]> { A, B(Option<&'a P>) }
+//~^ ERROR type parameters cannot use `Self` in their defaults [E0735]
+
+union Usized<'a, P: Sized = [Self]> { x: i32, y: Option<&'a P> }
+//~^ ERROR type parameters cannot use `Self` in their defaults [E0735]
+
+fn demo_usages() {
+    // An ICE means you only get the error from the first line of the
+    // demo; comment each out to observe the other ICEs when trying
+    // this out on older versions of Rust.
+
+    let _ice: Snobound;
+    let _ice: Enobound;
+    let _ice: Unobound;
+    let _ice: Ssized;
+    let _ice: Esized;
+    let _ice: Usized;
+}

src/test/ui/generics/issue-61631-default-type-param-cannot-reference-self.stderr

@@ -0,0 +1,39 @@
+error[E0735]: type parameters cannot use `Self` in their defaults
+  --> $DIR/issue-61631-default-type-param-cannot-reference-self.rs:13:25
+   |
+LL | struct Snobound<'a, P = Self> { x: Option<&'a P> }
+   |                         ^^^^ `Self` in type parameter default
+
+error[E0735]: type parameters cannot use `Self` in their defaults
+  --> $DIR/issue-61631-default-type-param-cannot-reference-self.rs:16:23
+   |
+LL | enum Enobound<'a, P = Self> { A, B(Option<&'a P>) }
+   |                       ^^^^ `Self` in type parameter default
+
+error[E0735]: type parameters cannot use `Self` in their defaults
+  --> $DIR/issue-61631-default-type-param-cannot-reference-self.rs:19:24
+   |
+LL | union Unobound<'a, P = Self> { x: i32, y: Option<&'a P> }
+   |                        ^^^^ `Self` in type parameter default
+
+error[E0735]: type parameters cannot use `Self` in their defaults
+  --> $DIR/issue-61631-default-type-param-cannot-reference-self.rs:25:31
+   |
+LL | struct Ssized<'a, P: Sized = [Self]> { x: Option<&'a P> }
+   |                               ^^^^ `Self` in type parameter default
+
+error[E0735]: type parameters cannot use `Self` in their defaults
+  --> $DIR/issue-61631-default-type-param-cannot-reference-self.rs:28:29
+   |
+LL | enum Esized<'a, P: Sized = [Self]> { A, B(Option<&'a P>) }
+   |                             ^^^^ `Self` in type parameter default
+
+error[E0735]: type parameters cannot use `Self` in their defaults
+  --> $DIR/issue-61631-default-type-param-cannot-reference-self.rs:31:30
+   |
+LL | union Usized<'a, P: Sized = [Self]> { x: i32, y: Option<&'a P> }
+   |                              ^^^^ `Self` in type parameter default
+
+error: aborting due to 6 previous errors
+
+For more information about this error, try `rustc --explain E0735`.