Add test for issue-59435

src/test/ui/specialization/issue-59435.rs

@@ -0,0 +1,17 @@
+#![feature(specialization)]
+#![allow(incomplete_features)]
+
+struct MyStruct {}
+
+trait MyTrait {
+    type MyType: Default;
+}
+
+impl MyTrait for i32 {
+    default type MyType = MyStruct;
+    //~^ ERROR: the trait bound `MyStruct: std::default::Default` is not satisfied
+}
+
+fn main() {
+    let _x: <i32 as MyTrait>::MyType = <i32 as MyTrait>::MyType::default();
+}

src/test/ui/specialization/issue-59435.stderr

@@ -0,0 +1,12 @@
+error[E0277]: the trait bound `MyStruct: std::default::Default` is not satisfied
+  --> $DIR/issue-59435.rs:11:5
+   |
+LL |     type MyType: Default;
+   |     --------------------- required by `MyTrait::MyType`
+...
+LL |     default type MyType = MyStruct;
+   |     ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ the trait `std::default::Default` is not implemented for `MyStruct`
+
+error: aborting due to previous error
+
+For more information about this error, try `rustc --explain E0277`.