fix various Match statement visitation errors (#1161)

Fixes #1160.

This PR also

- fixes `whitespace_before_colon` being swallowed during visitation on `MatchCase`s
- adds a new type of roundtrip test that catches issues of this class: the test applies a noop transformer to exercise the visitation API and compares the result with the original source.
- adds a few more cases to the match fixture

libcst/_nodes/statement.py

@@ -2854,17 +2854,16 @@ def _visit_and_replace_children(self, visitor: CSTVisitorT) -> "CSTNode":
                 self, "whitespace_after_case", self.whitespace_after_case, visitor
             ),
             pattern=visit_required(self, "pattern", self.pattern, visitor),
-            # pyre-fixme[6]: Expected `SimpleWhitespace` for 4th param but got
-            #  `Optional[SimpleWhitespace]`.
-            whitespace_before_if=visit_optional(
+            whitespace_before_if=visit_required(
                 self, "whitespace_before_if", self.whitespace_before_if, visitor
             ),
-            # pyre-fixme[6]: Expected `SimpleWhitespace` for 5th param but got
-            #  `Optional[SimpleWhitespace]`.
-            whitespace_after_if=visit_optional(
+            whitespace_after_if=visit_required(
                 self, "whitespace_after_if", self.whitespace_after_if, visitor
             ),
             guard=visit_optional(self, "guard", self.guard, visitor),
+            whitespace_before_colon=visit_required(
+                self, "whitespace_before_colon", self.whitespace_before_colon, visitor
+            ),
             body=visit_required(self, "body", self.body, visitor),
         )
 
@@ -3382,6 +3381,7 @@ def _visit_and_replace_children(self, visitor: CSTVisitorT) -> "MatchClass":
             whitespace_after_kwds=visit_required(
                 self, "whitespace_after_kwds", self.whitespace_after_kwds, visitor
             ),
+            rpar=visit_sequence(self, "rpar", self.rpar, visitor),
         )
 
     def _codegen_impl(self, state: CodegenState) -> None:

libcst/tests/test_roundtrip.py

@@ -3,25 +3,43 @@
 # This source code is licensed under the MIT license found in the
 # LICENSE file in the root directory of this source tree.
 
+
 from pathlib import Path
 from unittest import TestCase
 
-from libcst import parse_module
+from libcst import CSTTransformer, parse_module
 from libcst._parser.entrypoints import is_native
 
 fixtures: Path = Path(__file__).parent.parent.parent / "native/libcst/tests/fixtures"
 
 
+class NOOPTransformer(CSTTransformer):
+    pass
+
+
 class RoundTripTests(TestCase):
-    def test_clean_roundtrip(self) -> None:
+    def _get_fixtures(self) -> list[Path]:
         if not is_native():
             self.skipTest("pure python parser doesn't work with this")
         self.assertTrue(fixtures.exists(), f"{fixtures} should exist")
         files = list(fixtures.iterdir())
         self.assertGreater(len(files), 0)
-        for file in files:
+        return files
+
+    def test_clean_roundtrip(self) -> None:
+        for file in self._get_fixtures():
             with self.subTest(file=str(file)):
                 src = file.read_text(encoding="utf-8")
                 mod = parse_module(src)
                 self.maxDiff = None
                 self.assertEqual(mod.code, src)
+
+    def test_transform_roundtrip(self) -> None:
+        transformer = NOOPTransformer()
+        self.maxDiff = None
+        for file in self._get_fixtures():
+            with self.subTest(file=str(file)):
+                src = file.read_text(encoding="utf-8")
+                mod = parse_module(src)
+                new_mod = mod.visit(transformer)
+                self.assertEqual(src, new_mod.code)

native/libcst/tests/fixtures/malicious_match.py

@@ -37,4 +37,6 @@
     case x,y  ,  * more   :pass
     case y.z: pass
     case 1, 2: pass
+    case ( Foo  (   )    ) : pass
+    case (lol)  if (  True , )  :pass