Reverse the order of positional args check in _check_first_arg_for_type

Because the positional only arguments can appear before the normal positional
or keyword arguments, we need to first check the positional only ones for
the purpose of bad-mcs-* and similar methods.

Close #3230

pylint/checkers/classes.py

@@ -1477,10 +1477,10 @@ def _check_first_arg_for_type(self, node, metaclass=0):
         # don't care about functions with unknown argument (builtins)
         if node.args.args is None:
             return
-        if node.args.args:
-            first_arg = node.argnames()[0]
-        elif node.args.posonlyargs:
+        if node.args.posonlyargs:
             first_arg = node.args.posonlyargs[0].name
+        elif node.args.args:
+            first_arg = node.argnames()[0]
         else:
             first_arg = None
         self._first_attrs.append(first_arg)

tests/functional/r/regression_posonly_args.py

@@ -0,0 +1,19 @@
+"""Test linting.
+
+This is only intended to test pylint support for python 3.8's
+positional-only arguments (PEP 570).
+"""
+
+
+class Foobar:
+    """Class for frobulating the Foobar."""
+
+    @classmethod
+    def buildme(cls, /, value):
+        """Construct object using alternate method."""
+        return cls(value).abc
+
+    def runme(self, qrs, /, xyz=None):
+        """Do something funcy."""
+        if self.abc and qrs and xyz:
+            print("found something else")

tests/functional/r/regression_posonly_args.rc

@@ -0,0 +1,2 @@
+[testoptions]
+min_pyver=3.8