Add tests and news fragment

Fixes omry#572

news/572.api_change

@@ -0,0 +1,2 @@
+Implicitly empty resolver arguments (e.g., `${foo:a,}`) are deprecated in favor of explicit quoted strings (e.g., `${foo:a,""}`)
+

omegaconf/grammar_visitor.py

@@ -250,12 +250,13 @@ def visitSequence(
         # (element (COMMA element?)*) | (COMMA element?)+
         assert ctx.getChildCount() >= 1
 
+        # DEPRECATED: remove in 2.2 (revert #571)
         def empty_str_warning() -> None:
             txt = ctx.getText()
             warnings.warn(
                 f"In the sequence `{txt}` some elements are missing: please replace "
-                f"them with empty strings (\"\" or '') since this will not be supported "
-                f"anymore in the future.",
+                f"them with empty quoted strings. "
+                f"See https://github.com/omry/omegaconf/issues/572 for details.",
                 category=UserWarning,
             )
 

tests/test_grammar.py

@@ -1,8 +1,9 @@
 import math
+import re
 from typing import Any, Callable, List, Optional, Tuple
 
 import antlr4
-from pytest import mark, param, raises
+from pytest import mark, param, raises, warns
 
 from omegaconf import (
     DictConfig,
@@ -369,6 +370,30 @@ def test_config_value(
         parse_tree, expected_visit = self._parse("configValue", definition, expected)
         self._visit_with_config(parse_tree, expected_visit)
 
+    @parametrize_from(
+        [
+            ("trailing_comma", "${test:a,b,}", ["a", "b", ""]),
+            ("empty_middle", "${test:a,,b}", ["a", "", "b"]),
+            ("empty_first", "${test:,a,b}", ["", "a", "b"]),
+            ("single_comma", "${test:,}", ["", ""]),
+            (
+                "mixed_with_ws",
+                "${test:  ,a,b,\t,,c,  \t   \t ,d,, \t}",
+                ["", "a", "b", "", "", "c", "", "d", "", ""],
+            ),
+        ]
+    )
+    def test_deprecated_empty_args(
+        self, restore_resolvers: Any, definition: str, expected: Any
+    ) -> None:
+        OmegaConf.register_new_resolver("test", self._resolver_test)
+
+        parse_tree, expected_visit = self._parse("singleElement", definition, expected)
+        with warns(
+            UserWarning, match=re.escape("https://github.com/omry/omegaconf/issues/572")
+        ):
+            self._visit_with_config(parse_tree, expected_visit)
+
     def _check_is_same_type(self, value: Any, expected: Any) -> None:
         """
         Helper function to validate that types of `value` and `expected are the same.