Add regression test for #79224

Signed-off-by: Yuki Okushi <[email protected]>

src/test/ui/specialization/min_specialization/issue-79224.rs

@@ -0,0 +1,24 @@
+#![feature(min_specialization)]
+use std::fmt::{self, Display};
+
+pub enum Cow<'a, B: ?Sized + 'a, O = <B as ToOwned>::Owned>
+where
+    B: ToOwned,
+{
+    Borrowed(&'a B),
+    Owned(O),
+}
+
+impl ToString for Cow<'_, str> {
+    fn to_string(&self) -> String {
+        String::new()
+    }
+}
+
+impl<B: ?Sized> Display for Cow<'_, B> { //~ ERROR: the trait bound `B: Clone` is not satisfied [E0277]
+    fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result { //~ ERROR: the trait bound `B: Clone` is not satisfied [E0277]
+        write!(f, "foo")
+    }
+}
+
+fn main() {}

src/test/ui/specialization/min_specialization/issue-79224.stderr

@@ -0,0 +1,29 @@
+error[E0277]: the trait bound `B: Clone` is not satisfied
+  --> $DIR/issue-79224.rs:18:17
+   |
+LL | impl<B: ?Sized> Display for Cow<'_, B> {
+   |                 ^^^^^^^ the trait `Clone` is not implemented for `B`
+   |
+   = note: required because of the requirements on the impl of `ToOwned` for `B`
+help: consider further restricting this bound
+   |
+LL | impl<B: ?Sized + std::clone::Clone> Display for Cow<'_, B> {
+   |                +++++++++++++++++++
+
+error[E0277]: the trait bound `B: Clone` is not satisfied
+  --> $DIR/issue-79224.rs:19:5
+   |
+LL | /     fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
+LL | |         write!(f, "foo")
+LL | |     }
+   | |_____^ the trait `Clone` is not implemented for `B`
+   |
+   = note: required because of the requirements on the impl of `ToOwned` for `B`
+help: consider further restricting this bound
+   |
+LL | impl<B: ?Sized + std::clone::Clone> Display for Cow<'_, B> {
+   |                +++++++++++++++++++
+
+error: aborting due to 2 previous errors
+
+For more information about this error, try `rustc --explain E0277`.