diff --git a/news/5827.feature b/news/5827.feature
new file mode 100644
index 00000000000..2ef8d45be1f
--- /dev/null
+++ b/news/5827.feature
@@ -0,0 +1 @@
+A warning message is emitted when dropping an ``--[extra-]index-url`` value that points to an existing local directory.
diff --git a/src/pip/_internal/index.py b/src/pip/_internal/index.py
index 9eb8e120751..6d38e1240b6 100644
--- a/src/pip/_internal/index.py
+++ b/src/pip/_internal/index.py
@@ -216,6 +216,11 @@ def sort_path(path):
                             sort_path(os.path.join(path, item))
                     elif is_file_url:
                         urls.append(url)
+                    else:
+                        logger.warning(
+                            "Path '{0}' is ignored: "
+                            "it is a directory.".format(path),
+                        )
                 elif os.path.isfile(path):
                     sort_path(path)
                 else: