Add as_string visitor for Unknown node (#1268)

ChangeLog

@@ -10,6 +10,10 @@ Release date: TBA
 
 * Always treat ``__class_getitem__`` as a classmethod.
 
+* Add missing ``as_string`` visitor method for ``Unknown`` node.
+
+  Closes #1264
+
 
 What's New in astroid 2.8.6?
 ============================

astroid/nodes/as_string.py

@@ -36,6 +36,7 @@
         MatchSingleton,
         MatchStar,
         MatchValue,
+        Unknown,
     )
 
 # pylint: disable=unused-argument
@@ -643,6 +644,9 @@ def visit_property(self, node):
     def visit_evaluatedobject(self, node):
         return node.original.accept(self)
 
+    def visit_unknown(self, node: "Unknown") -> str:
+        return str(node)
+
 
 def _import_string(names):
     """return a list of (name, asname) formatted as a string"""

tests/unittest_nodes.py

@@ -306,6 +306,11 @@ def test_f_strings(self):
         ast = abuilder.string_build(code)
         self.assertEqual(ast.as_string().strip(), code.strip())
 
+    @staticmethod
+    def test_as_string_unknown() -> None:
+        assert nodes.Unknown().as_string() == "Unknown.Unknown()"
+        assert nodes.Unknown(lineno=1, col_offset=0).as_string() == "Unknown.Unknown()"
+
 
 class _NodeTest(unittest.TestCase):
     """test transformation of If Node"""