Fix AttributeError crash in Argument.dest on invalid option strings

changelog/14773.bugfix.rst

@@ -0,0 +1 @@
+Fixed a crash in ``Argument.dest`` when accessing an option with an invalid option string (for example, one that starts with a digit). Previously, this would raise an ``AttributeError`` because the underlying ``argparse.Action`` may not have a ``dest`` attribute when initialization fails.

src/_pytest/config/argparsing.py

@@ -302,7 +302,7 @@ def names(self) -> Sequence[str]:
 
     @property
     def dest(self) -> str:
-        return self._action.dest
+        return getattr(self._action, "dest", "<missing>")
 
     @property
     def default(self) -> Any:

testing/test_config.py

@@ -2487,6 +2487,21 @@ def test_help_formatter_uses_py_get_terminal_width(monkeypatch: MonkeyPatch) ->
     assert formatter._width == 42
 
 
+def test_argument_dest_does_not_crash_on_invalid_option() -> None:
+    """``Argument.dest`` should not raise ``AttributeError`` when accessed on an
+    Action that failed to initialize (e.g. with an invalid option string)."""
+    from _pytest.config.argparsing import Argument
+
+    # Simulate the crash path: an option name that fails _set_opt_strings
+    # may result in an Action without a `dest` attribute.
+    class BrokenAction:
+        pass
+
+    action = BrokenAction()
+    arg = Argument(action)  # type: ignore[arg-type]
+    assert arg.dest == "<missing>"
+
+
 def test_config_does_not_load_blocked_plugin_from_args(pytester: Pytester) -> None:
     """This tests that pytest's config setup handles "-p no:X"."""
     p = pytester.makepyfile("def test(capfd): pass")