[3.12] GH-105588: Add missing error checks to some obj2ast_* converters (GH-105838)

GH-105588: Add missing error checks to some obj2ast_* converters (GH-105589)
(cherry picked from commit a4056c8f9c)

Co-authored-by: Brandt Bucher <brandtbucher@microsoft.com>

Lib/test/test_ast.py

@@ -3,6 +3,7 @@ import builtins
 import dis
 import enum
 import os
+import re
 import sys
 import textwrap
 import types
@@ -1110,6 +1111,32 @@ class AST_Tests(unittest.TestCase):
             msg="source code string cannot contain null bytes"):
             ast.parse("a\0b")
 
+    def assert_none_check(self, node: type[ast.AST], attr: str, source: str) -> None:
+        with self.subTest(f"{node.__name__}.{attr}"):
+            tree = ast.parse(source)
+            found = 0
+            for child in ast.walk(tree):
+                if isinstance(child, node):
+                    setattr(child, attr, None)
+                    found += 1
+            self.assertEqual(found, 1)
+            e = re.escape(f"field '{attr}' is required for {node.__name__}")
+            with self.assertRaisesRegex(ValueError, f"^{e}$"):
+                compile(tree, "<test>", "exec")
+
+    def test_none_checks(self) -> None:
+        tests = [
+            (ast.alias, "name", "import spam as SPAM"),
+            (ast.arg, "arg", "def spam(SPAM): spam"),
+            (ast.comprehension, "target", "[spam for SPAM in spam]"),
+            (ast.comprehension, "iter", "[spam for spam in SPAM]"),
+            (ast.keyword, "value", "spam(**SPAM)"),
+            (ast.match_case, "pattern", "match spam:\n case SPAM: spam"),
+            (ast.withitem, "context_expr", "with SPAM: spam"),
+        ]
+        for node, attr, source in tests:
+            self.assert_none_check(node, attr, source)
+
 class ASTHelpers_Test(unittest.TestCase):
     maxDiff = None