Add a couple more tests

src/test/ui/generic-associated-types/issue-87429-associated-type-default.rs

@@ -0,0 +1,18 @@
+// check-fail
+
+#![feature(associated_type_defaults)]
+#![feature(generic_associated_types)]
+
+trait Family {
+    // Fine, i32: PartialEq<i32>
+    type Member<'a>: for<'b> PartialEq<Self::Member<'b>> = i32;
+}
+
+struct Foo;
+trait Family2 {
+    // Not fine, not Foo: PartialEq<Foo>
+    type Member<'a>: for<'b> PartialEq<Self::Member<'b>> = Foo;
+    //~^ ERROR can't compare
+}
+
+fn main() {}

src/test/ui/generic-associated-types/issue-87429-associated-type-default.stderr

@@ -0,0 +1,14 @@
+error[E0277]: can't compare `Foo` with `Foo`
+  --> $DIR/issue-87429-associated-type-default.rs:14:5
+   |
+LL |     type Member<'a>: for<'b> PartialEq<Self::Member<'b>> = Foo;
+   |     ^^^^^^^^^^^^^^^^^-----------------------------------^^^^^^^
+   |     |                |
+   |     |                required by this bound in `Family2::Member`
+   |     no implementation for `Foo == Foo`
+   |
+   = help: the trait `PartialEq` is not implemented for `Foo`
+
+error: aborting due to previous error
+
+For more information about this error, try `rustc --explain E0277`.

src/test/ui/generic-associated-types/issue-87429-specialization.rs

@@ -0,0 +1,25 @@
+// check-fail
+
+#![feature(specialization)]
+//~^ WARN incomplete
+#![feature(generic_associated_types)]
+
+trait Family {
+    type Member<'a>: for<'b> PartialEq<Self::Member<'b>>;
+}
+
+struct I32Family;
+
+impl Family for I32Family {
+    default type Member<'a> = i32;
+}
+
+struct Foo;
+struct FooFamily;
+
+impl Family for FooFamily {
+    default type Member<'a> = Foo;
+    //~^ ERROR can't compare
+}
+
+fn main() {}

src/test/ui/generic-associated-types/issue-87429-specialization.stderr

@@ -0,0 +1,24 @@
+warning: the feature `specialization` is incomplete and may not be safe to use and/or cause compiler crashes
+  --> $DIR/issue-87429-specialization.rs:3:12
+   |
+LL | #![feature(specialization)]
+   |            ^^^^^^^^^^^^^^
+   |
+   = note: `#[warn(incomplete_features)]` on by default
+   = note: see issue #31844 <https://github.com/rust-lang/rust/issues/31844> for more information
+   = help: consider using `min_specialization` instead, which is more stable and complete
+
+error[E0277]: can't compare `Foo` with `Foo`
+  --> $DIR/issue-87429-specialization.rs:21:5
+   |
+LL |     type Member<'a>: for<'b> PartialEq<Self::Member<'b>>;
+   |                      ----------------------------------- required by this bound in `Family::Member`
+...
+LL |     default type Member<'a> = Foo;
+   |     ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ no implementation for `Foo == Foo`
+   |
+   = help: the trait `PartialEq` is not implemented for `Foo`
+
+error: aborting due to previous error; 1 warning emitted
+
+For more information about this error, try `rustc --explain E0277`.