throw if addoption called after preparse with no default

AUTHORS

@@ -185,6 +185,7 @@ Katerina Koukiou
 Keri Volans
 Kevin Cox
 Kevin J. Foley
+Kevin Santana
 Kodi B. Arfer
 Kostis Anagnostopoulos
 Kristoffer Nordström

changelog/9303.bugfix.rst

@@ -0,0 +1,2 @@
+`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.

src/_pytest/config/argparsing.py

@@ -362,6 +362,11 @@ class OptionGroup:
         results in help showing ``--two-words`` only, but ``--twowords`` gets
         accepted **and** the automatic destination is in ``args.twowords``.
         """
+        if getattr(self.parser, "after_preparse", False) and "default" not in attrs:
+            raise ValueError(
+                "Cannot add options without default after initial conftest discovery"
+            )
+
         conflict = set(optnames).intersection(
             name for opt in self.options for name in opt.names()
         )

testing/test_pluginmanager.py

@@ -78,6 +78,21 @@ class TestPytestPluginInteractions:
         )
         assert config.option.test123
 
+    def test_do_option_postinit_nodefault(self, pytester: Pytester) -> None:
+        config = pytester.parseconfigure()
+        assert not hasattr(config.option, "deadbeef")
+        p = pytester.makepyfile(
+            """
+            def pytest_addoption(parser):
+                parser.addoption('--deadbeef', action="store_true")
+        """
+        )
+        with pytest.raises(ValueError):
+            config.pluginmanager._importconftest(
+                p, importmode="prepend", rootpath=pytester.path
+            )
+        assert not hasattr(config.option, "deadbeef")
+
     def test_configure(self, pytester: Pytester) -> None:
         config = pytester.parseconfig()
         values = []