Add test for issue-39618

Author: JohnTitor

src/test/ui/specialization/issue-39618.rs

@@ -0,0 +1,24 @@
+// Regression test for #39618, shouldn't crash.
+// check-pass
+
+#![feature(specialization)]
+
+trait Foo {
+    fn foo(&self);
+}
+
+trait Bar {
+    fn bar(&self);
+}
+
+impl<T> Bar for T where T: Foo {
+    fn bar(&self) {}
+}
+
+impl<T> Foo for T where T: Bar {
+    fn foo(&self) {}
+}
+
+impl Foo for u64 {}
+
+fn main() {}