Consider paths and pathlists relative to cwd in case of an absent ini-file (#11963)

Previously this would trigger an `AssertionError`.

While it could be considered a bug-fix, but given it now can be relied upon, it is probably better to consider it an improvement.

Fix #11311

changelog/11311.improvement.rst

@@ -0,0 +1,4 @@
+When using ``--override-ini`` for paths in invocations without a configuration file defined, the current working directory is used
+as the relative directory.
+
+Previoulsy this would raise an :class:`AssertionError`.

src/_pytest/config/__init__.py

@@ -1563,9 +1563,11 @@ class Config:
         #   in this case, we already have a list ready to use.
         #
         if type == "paths":
-            # TODO: This assert is probably not valid in all cases.
-            assert self.inipath is not None
-            dp = self.inipath.parent
+            dp = (
+                self.inipath.parent
+                if self.inipath is not None
+                else self.invocation_params.dir
+            )
             input_values = shlex.split(value) if isinstance(value, str) else value
             return [dp / x for x in input_values]
         elif type == "args":

src/_pytest/config/argparsing.py

@@ -198,9 +198,16 @@ class Parser:
                 * ``paths``: a list of :class:`pathlib.Path`, separated as in a shell
                 * ``pathlist``: a list of ``py.path``, separated as in a shell
 
+            For ``paths`` and ``pathlist`` types, they are considered relative to the ini-file.
+            In case the execution is happening without an ini-file defined,
+            they will be considered relative to the current working directory (for example with ``--override-ini``).
+
             .. versionadded:: 7.0
                 The ``paths`` variable type.
 
+            .. versionadded:: 8.1
+                Use the current working directory to resolve ``paths`` and ``pathlist`` in the absence of an ini-file.
+
             Defaults to ``string`` if ``None`` or not passed.
         :param default:
             Default value if no ini-file option exists but is queried.

testing/test_config.py

@@ -1874,6 +1874,18 @@ class TestOverrideIniArgs:
         assert "ERROR:" not in result.stderr.str()
         result.stdout.fnmatch_lines(["collected 1 item", "*= 1 passed in *="])
 
+    def test_override_ini_without_config_file(self, pytester: Pytester) -> None:
+        pytester.makepyfile(**{"src/override_ini_without_config_file.py": ""})
+        pytester.makepyfile(
+            **{
+                "tests/test_override_ini_without_config_file.py": (
+                    "import override_ini_without_config_file\ndef test(): pass"
+                ),
+            }
+        )
+        result = pytester.runpytest("--override-ini", "pythonpath=src")
+        assert result.parseoutcomes() == {"passed": 1}
+
 
 def test_help_via_addopts(pytester: Pytester) -> None:
     pytester.makeini(