Don't rely on implicit fallthrough in `switch` statements

Reason

Always end a non-empty case with a break. Accidentally leaving out a break is a fairly common bug. A deliberate fallthrough can be a maintenance hazard and should be rare and explicit.

Example

switch (eventType) {
case Information:
    update_status_bar();
    break;
case Warning:
    write_event_log();
    // Bad - implicit fallthrough
case Error:
    display_error_window();
    break;
}

Multiple case labels of a single statement is OK:

switch (x) {
case 'a':
case 'b':
case 'f':
    do_something(x);
    break;
}

Return statements in a case label are also OK:

switch (x) {
case 'a':
    return 1;
case 'b':
    return 2;
case 'c':
    return 3;
}

Exceptions

In rare cases if fallthrough is deemed appropriate, be explicit and use the [[fallthrough]] annotation:

switch (eventType) {
case Information:
    update_status_bar();
    break;
case Warning:
    write_event_log();
    [[fallthrough]];
case Error:
    display_error_window();
    break;
}

Note

Enforcement

Flag all implicit fallthroughs from non-empty cases.