cpp/language/attributes/fallthrough

Indicates that the fall through from the previous case label is intentional and should not be diagnosed by a compiler that warns on fallthrough.

Explanation
May only be applied to a null statement to create a fallthrough statement.

A fallthrough statement may only be used in a switch statement, where the next statement to be executed is a statement with a case or default label for that switch statement. If the fallthrough statement is inside a loop, the next (labeled) statement must be part of the same iteration of that loop.