-fsanitize=undefined confirms there is no overflow.

This statement is false. (Speaking as a logician ;-) )

-fsanitize=xxx instruments the code with run-time checks. Of course, just compiling with -fsanitize=undefined does not provide any confirmation/infirmation on the potential overflow.

If you run the instrumented code, and it aborts or logs "UB: overflow", you can definitely state that there is an overflow. If you get no error, it just means that:

• either no overflow is possible at all
• or the inputs you provided did not trigger an overflow, but others would have.

Cheers !