Improve AST safety check

Fixes psf#4270, regressed by psf#4270

CHANGES.md

@@ -9,6 +9,7 @@
 ### Stable style
 
 <!-- Changes that affect Black's stable style -->
+- Fix unwanted crashes caused by AST equivalency check (#4290)
 
 ### Preview style
 

src/black/parsing.py

@@ -223,11 +223,9 @@ def _stringify_ast(node: ast.AST, parent_stack: List[ast.AST]) -> Iterator[str]:
                 and field == "value"
                 and isinstance(value, str)
                 and len(parent_stack) >= 2
+                # Any standalone string, ideally this would
+                # exactly match black.nodes.is_docstring
                 and isinstance(parent_stack[-1], ast.Expr)
-                and isinstance(
-                    parent_stack[-2],
-                    (ast.FunctionDef, ast.AsyncFunctionDef, ast.Module, ast.ClassDef),
-                )
             ):
                 # Constant strings may be indented across newlines, if they are
                 # docstrings; fold spaces after newlines when comparing. Similarly,

tests/test_black.py

@@ -2908,6 +2908,22 @@ async def f():
                 """docstring"""
             ''',
         )
+        self.check_ast_equivalence(
+            """
+            if __name__ == "__main__":
+                "  docstring-like  "
+            """,
+            '''
+            if __name__ == "__main__":
+                """docstring-like"""
+            ''',
+        )
+        self.check_ast_equivalence(r'def f(): r" \n "', r'def f(): "\\n"')
+        self.check_ast_equivalence('try: pass\nexcept: " x "', 'try: pass\nexcept: "x"')
+
+        self.check_ast_equivalence(
+            'def foo(): return " x "', 'def foo(): return "x"', should_fail=True
+        )
 
     def test_assert_equivalent_fstring(self) -> None:
         major, minor = sys.version_info[:2]