cp: backup dest symlink linking to source

src/uu/cp/src/cp.rs

@@ -1521,6 +1521,7 @@ fn is_forbidden_copy_to_same_file(
         options.dereference(source_in_command_line) || !source.is_symlink();
     paths_refer_to_same_file(source, dest, dereference_to_compare)
         && !(options.force() && options.backup != BackupMode::NoBackup)
+        && !(dest.is_symlink() && options.backup != BackupMode::NoBackup)
 }
 
 /// Back up, remove, or leave intact the destination file, depending on the options.

tests/by-util/test_cp.rs

@@ -701,6 +701,25 @@ fn test_cp_arg_backup_with_dest_a_symlink() {
     assert_eq!(original, at.resolve_link(backup));
 }
 
+#[test]
+fn test_cp_arg_backup_with_dest_a_symlink_to_source() {
+    let (at, mut ucmd) = at_and_ucmd!();
+    let source = "source";
+    let source_content = "content";
+    let symlink = "symlink";
+    let backup = "symlink~";
+
+    at.write(source, source_content);
+    at.symlink_file(source, symlink);
+
+    ucmd.arg("-b").arg(source).arg(symlink).succeeds();
+
+    assert!(!at.symlink_exists(symlink));
+    assert_eq!(source_content, at.read(symlink));
+    assert!(at.symlink_exists(backup));
+    assert_eq!(source, at.resolve_link(backup));
+}
+
 #[test]
 fn test_cp_arg_backup_with_other_args() {
     let (at, mut ucmd) = at_and_ucmd!();