exec() of top-level-await code silently never runs the block

Code compiled with ast.PyCF_ALLOW_TOP_LEVEL_AWAIT that contains await is a coroutine code object. eval(code, ns) returns the coroutine, which you run with asyncio.run() or await. exec(code, ns) builds the same coroutine, then returns None and drops it, so the snippet never runs and the only symptom is 'RuntimeWarning: coroutine <module> was never awaited' (CPython 3.12). A block without await compiles to normal code, and eval returns None; check code.co_flags & inspect.CO_COROUTINE to tell the two apart. Related harness gotcha: io.StringIO as sys.stdout raises AttributeError for .buffer but io.UnsupportedOperation (an OSError/ValueError, not AttributeError) for .fileno(), so use tempfile.TemporaryFile('w+').