Add regression test for inference failures

tests/ui/traits/generic-cow-inference-regression.rs

@@ -0,0 +1,20 @@
+//@ run-pass
+
+// regression test for #147964:
+//   constification of these traits resulted in inference errors due to additional where clauses
+
+use std::borrow::{Cow, Borrow};
+
+pub fn generic_deref<'a, T: ToOwned<Owned = U>, U>(cow: Cow<'a, T>) {
+    let _: &T = &cow;
+}
+
+pub fn generic_borrow<'a, T: ToOwned<Owned = U>, U>(cow: Cow<'a, T>) {
+    let _: &T = cow.borrow();
+}
+
+pub fn generic_as_ref<'a, T: ToOwned<Owned = U>, U>(cow: Cow<'a, T>) {
+    let _: &T = cow.as_ref();
+}
+
+fn main() {}