Fix annotation of posonlyarg (#508)

Author: asottile

pyflakes/checker.py

@@ -1799,6 +1799,10 @@ def addArgs(arglist):
             addArgs(node.args.args)
             defaults = node.args.defaults
         else:
+            if PY38_PLUS:
+                for arg in node.args.posonlyargs:
+                    args.append(arg.arg)
+                    annotations.append(arg.annotation)
             for arg in node.args.args + node.args.kwonlyargs:
                 args.append(arg.arg)
                 annotations.append(arg.annotation)

pyflakes/test/test_type_annotations.py

@@ -418,3 +418,11 @@ def t(self) -> Y:
                 Y = 2
                 return Y
         """, m.UndefinedName)
+
+    @skipIf(version_info < (3, 8), 'new in Python 3.8')
+    def test_positional_only_argument_annotations(self):
+        self.flakes("""
+        from x import C
+
+        def f(c: C, /): ...
+        """)