Add strict_warnings mode for internal pytest warnings

src/_pytest/config/argparsing.py

@@ -443,6 +443,18 @@ def __init__(
     def error(self, message: str) -> NoReturn:
         """Transform argparse error message into UsageError."""
         msg = f"{self.prog}: error: {message}"
+        if "unrecognized arguments:" in message:
+            file_or_dir_args = getattr(self._parser, "extra_info", {}).get(
+                "file_or_dir", []
+            )
+            if file_or_dir_args:
+                from pathlib import Path
+
+                missing_paths = [
+                    str(p) for p in file_or_dir_args if not Path(str(p)).exists()
+                ]
+                if missing_paths:
+                    msg += "\nNote: The specified path(s) do not exist, so custom CLI options from conftest.py may not be available."
 
         if hasattr(self._parser, "_config_source_hint"):
             msg = f"{msg} ({self._parser._config_source_hint})"

src/_pytest/main.py

@@ -106,6 +106,19 @@ def pytest_addoption(parser: Parser) -> None:
         "Processed after -W/--pythonwarnings.",
     )
 
+    parser.addini(
+        "strict_warnings",
+        type="bool",
+        default=False,
+        help="If true, treat internal pytest warnings (PytestWarning and subclasses) as errors.",
+    )
+    group.addoption(
+        "--strict-warnings",
+        action="store_true",
+        dest="strict_warnings",
+        help="Treat internal pytest warnings (PytestWarning and subclasses) as errors.",
+    )
+
     group = parser.getgroup("collect", "collection")
     group.addoption(
         "--collectonly",

src/_pytest/warnings.py

@@ -53,6 +53,12 @@ def catch_warnings_for_item(
                 for arg in mark.args:
                     warnings.filterwarnings(*parse_warning_filter(arg, escape=False))
 
+        strict_warnings = bool(config.getini("strict_warnings")) or bool(
+            getattr(config.option, "strict_warnings", False)
+        )
+        if strict_warnings:
+            warnings.filterwarnings("error", category=pytest.PytestWarning)
+
         try:
             yield
         finally:

testing/test_main.py

@@ -271,6 +271,19 @@ def test_absolute_paths_are_resolved_correctly(self, invocation_path: Path) -> N
             module_name=None,
         )
 
+    def test_custom_cli_arg_with_missing_path(pytester: Pytester):
+        """Test that a helpful error message is shown when a custom CLI argument is used with a non-existent path."""
+        pytester.makeconftest(
+            """
+            def pytest_addoption(parser):
+                parser.addoption("--potato", default="")
+            """
+        )
+        result = pytester.runpytest("file_does_not_exist.py", "--potato=yum")
+        assert result.ret != 0
+        assert "unrecognized arguments: --potato=yum" in result.stderr.str()
+        assert "Note: The specified path(s) do not exist" in result.stderr.str()
+
 
 def test_module_full_path_without_drive(pytester: Pytester) -> None:
     """Collect and run test using full path except for the drive letter (#7628).

testing/test_warnings.py

@@ -590,6 +590,34 @@ def test_foo():
             result, "assertion is always true, perhaps remove parentheses?"
         )
 
+def test_strict_warnings_treats_pytest_collection_warning_as_error(
+    pytester: Pytester,
+) -> None:
+    """In strict warnings mode, PytestCollectionWarning should be treated as an error."""
+    pytester.makepyfile(
+        """
+        import pytest
+        pytestmark = pytest.mark.filterwarnings("default::pytest.PytestCollectionWarning")
+
+        class TestFoo:
+            def __new__(cls):
+                return super().__new__(cls)
+        """
+    )
+    result = pytester.runpytest("--collect-only")
+    result.stdout.fnmatch_lines(
+        [
+            "*warnings summary*",
+            "*PytestCollectionWarning: cannot collect test class 'TestFoo' because it has a __new__ constructor*",
+        ]
+    )
+    result_strict = pytester.runpytest("--collect-only", "--strict-warnings")
+    assert result_strict.ret != 0
+    result_strict.stdout.fnmatch_lines(
+        [
+            "*PytestCollectionWarning: cannot collect test class 'TestFoo' because it has a __new__ constructor*",
+        ]
+    )
 
 def test_warnings_checker_twice() -> None:
     """Issue #4617"""