Add a new test case to verify behavior.

src/test/ui/traits/trait-upcasting/correct-supertrait-substitution.rs

@@ -0,0 +1,39 @@
+// run-pass
+#![feature(trait_upcasting)]
+#![allow(incomplete_features)]
+
+trait Foo<T: Default + ToString>: Bar<i32> + Bar<T> {}
+trait Bar<T: Default + ToString> {
+    fn bar(&self) -> String {
+        T::default().to_string()
+    }
+}
+
+struct S1;
+
+impl Bar<i32> for S1 {}
+impl Foo<i32> for S1 {}
+
+struct S2;
+impl Bar<i32> for S2 {}
+impl Bar<bool> for S2 {}
+impl Foo<bool> for S2 {}
+
+fn test1(x: &dyn Foo<i32>) {
+    let s = x as &dyn Bar<i32>;
+    assert_eq!("0", &s.bar().to_string());
+}
+
+fn test2(x: &dyn Foo<bool>) {
+    let p = x as &dyn Bar<i32>;
+    assert_eq!("0", &p.bar().to_string());
+    let q = x as &dyn Bar<bool>;
+    assert_eq!("false", &q.bar().to_string());
+}
+
+fn main() {
+    let s1 = S1;
+    test1(&s1);
+    let s2 = S2;
+    test2(&s2);
+}