> 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