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> With 3.9 on Windows, using Benjamin's example, I do not get the Windows equivalent of a seg fault. However, execution stops at compile with no exception, including SystemExit.
I can still reproduce on Linux,
 $ python
Python 3.10.0a0 (heads/bpo-xxxxx:f2947e354c, May 21 2020, 18:54:57) 
[GCC 9.2.1 20191008] on linux
Type "help", "copyright", "credits" or "license" for more information.
>>> import ast
>>> e = ast.UnaryOp(op=ast.Not(), lineno=0, col_offset=0)
>>> e.operand = e
>>> compile(ast.Expression(e), "<test>", "eval")
[1] 15320 segmentation fault (core dumped) python
> These examples amount to limited fuzz testing of compile(). I think it should raise something like "SyntaxError: recursive ast" or even 'bad ast' if malformed non-recursive asts have the same issue.
I dont think it would be easy to locate such errors before they happen, instead I propose (actually already proposed in PR 20594) to add recursion guards to places where this might happen. This can prevent crashes on both direct and indirect cycles
>>> import ast
>>> e = ast.UnaryOp(op=ast.Not(), lineno=0, col_offset=0)
>>> e.operand = e
>>> compile(ast.Expression(e), "<test>", "eval")
Traceback (most recent call last):
 File "<stdin>", line 1, in <module>
RecursionError: maximum recursion depth exceeded while traversing 'expr' node
>>> e = ast.UnaryOp(op=ast.Not(), lineno=0, col_offset=0)
>>> f = ast.UnaryOp(op=ast.Not(), lineno=0, col_offset=0)
>>> e.operand = f
>>> f.operand = e
>>> compile(ast.Expression(e), "<test>", "eval")
Traceback (most recent call last):
 File "<stdin>", line 1, in <module>
RecursionError: maximum recursion depth exceeded while traversing 'expr' node