Clearer error when trying to use a relative interpolation

omegaconf/built_in_resolvers.py

@@ -152,11 +152,17 @@ def _get_and_validate_dict_input(
     if isinstance(in_dict, str):
         # Path to an existing key in the config: use `select()`.
         key = in_dict
+        if key.startswith("."):
+            raise NotImplementedError(
+                f"To use relative interpolations with `{resolver_name}`, please use "
+                f"the explicit interpolation syntax: ${{{resolver_name}:${{{key}}}}}"
+            )
         in_dict = OmegaConf.select(
             root, key, throw_on_missing=True, default=_DEFAULT_SELECT_MARKER_
         )
         if in_dict is _DEFAULT_SELECT_MARKER_:
             raise ConfigKeyError(f"Key not found: '{key}'")
+        assert in_dict is not None
 
     if not isinstance(in_dict, Mapping):
         raise TypeError(

tests/interpolation/built_in_resolvers/test_dict.py

@@ -104,6 +104,15 @@ def test_dict_keys(cfg: Any, key: Any, expected: Any) -> None:
             ),
             id="type_error_dictconfig",
         ),
+        param(
+            {"foo": "${oc.dict.keys_or_values:.bar}", "bar": {"x": 0}},
+            "foo",
+            raises(
+                InterpolationResolutionError,
+                match=re.escape("NotImplementedError"),
+            ),
+            id="relative_with_select",
+        ),
     ],
 )
 def test_get_and_validate_dict_input(