This is undefined behavior in my understanding, it just happens to work until it doesn't.

I wouldn't be surprised if any null check against this would be erased by the optimizer for example as the parent comment mentioned. Sanitizers might check for null this too.

Just to cite what the others have told you: https://godbolt.org/z/bWfaYrqoY

    /app/example.cpp:10:6: runtime error: member call on null pointer of type 'Foo'
    SUMMARY: UndefinedBehaviorSanitizer: undefined-behavior /app/example.cpp:10:6 
    /app/example.cpp:3:8: runtime error: member call on null pointer of type 'Foo *'
    SUMMARY: UndefinedBehaviorSanitizer: undefined-behavior /app/example.cpp:3:8

    Foo *p = nullptr;
    p->bar();

That's undefined behavior. It's "allowed" in the sense of "yes it's possible to write, compile and run that code", but the language makes no guarantees about what the results will be. So maybe not the most useful definition of "allowed".

That code is outside the set of valid c++ programs