warn user if addoption is used after configuration and parsing phase.

changelog/9303.bugfix.rst

@@ -1,2 +1,8 @@
 `Parser.addoption` now throws a value error if an attempt is made at adding an option
-after the pre_parsing process without including a default.
+after the pre_parsing process without including a default. If the user decides to
+provides a default a warning will be given since the option will not be available
+for use in the command line.
+
+This occurs if conftest files aren't in the root directory or any sub directory
+matching the `test*` regex. conftest files which match the previous statement
+will have their options added after the configuration and parsing phase.

src/_pytest/config/argparsing.py

@@ -22,6 +22,7 @@ from _pytest.deprecated import ARGUMENT_PERCENT_DEFAULT
 from _pytest.deprecated import ARGUMENT_TYPE_STR
 from _pytest.deprecated import ARGUMENT_TYPE_STR_CHOICE
 from _pytest.deprecated import check_ispytest
+from _pytest.warning_types import PytestConfigWarning
 
 if TYPE_CHECKING:
     from typing import NoReturn
@@ -363,9 +364,12 @@ class OptionGroup:
         accepted **and** the automatic destination is in ``args.twowords``.
         """
         if getattr(self.parser, "after_preparse", False) and "default" not in attrs:
-            raise ValueError(
+            raise UsageError("Cannot add options after initial conftest discovery")
-                "Cannot add options without default after initial conftest discovery"
+        elif getattr(self.parser, "after_preparse", False):
+            warn = ("Adding option {option} after configuration phase").format(
+                option=optnames
             )
+            warnings.warn(PytestConfigWarning(warn), stacklevel=3)
 
         conflict = set(optnames).intersection(
             name for opt in self.options for name in opt.names()

testing/test_pluginmanager.py

@@ -13,6 +13,7 @@ from _pytest.main import Session
 from _pytest.monkeypatch import MonkeyPatch
 from _pytest.pathlib import import_path
 from _pytest.pytester import Pytester
+from _pytest.warning_types import PytestConfigWarning
 
 
 @pytest.fixture
@@ -73,9 +74,13 @@ class TestPytestPluginInteractions:
                     default=True)
         """
         )
-        config.pluginmanager._importconftest(
+        with pytest.warns(PytestConfigWarning) as warn:
-            p, importmode="prepend", rootpath=pytester.path
+            config.pluginmanager._importconftest(
-        )
+                p, importmode="prepend", rootpath=pytester.path
+            )
+
+        assert len(warn) == 1
+        assert str(warn[0].message).endswith("after configuration phase")
         assert config.option.test123
 
     def test_do_option_postinit_nodefault(self, pytester: Pytester) -> None:
@@ -87,7 +92,9 @@ class TestPytestPluginInteractions:
                 parser.addoption('--deadbeef', action="store_true")
         """
         )
-        with pytest.raises(ValueError):
+        with pytest.raises(
+            UsageError, match="^Cannot add options after initial conftest discovery$"
+        ):
             config.pluginmanager._importconftest(
                 p, importmode="prepend", rootpath=pytester.path
             )