[Bug c++/81392] New: Improve diagnostics for [[fallthrough]] attribute that is missing a semicolon

["cody at codygray dot com" <gcc-bugzilla at gcc dot gnu dot org>]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=81392

            Bug ID: 81392
           Summary: Improve diagnostics for [[fallthrough]] attribute that
                    is missing a semicolon
           Product: gcc
           Version: 8.0
            Status: UNCONFIRMED
          Keywords: diagnostic
          Severity: normal
          Priority: P3
         Component: c++
          Assignee: unassigned at gcc dot gnu.org
          Reporter: cody at codygray dot com
  Target Milestone: ---

The C++1z/C++17 draft standard introduces a [[fallthrough]] attribute to
explicitly document that fall-through behavior is intended in a switch-case
block.

This works in conjunction with G++'s -Wimplicit-fallthrough option, which gives
a warning about potentially unintended fall-through behaviors.

The [[fallthrough]] attribute is required to be applied to an empty statement
(see http://www.open-std.org/jtc1/sc22/wg21/docs/papers/2016/p0188r1.pdf), and
therefore requires a terminating semicolon.

However, forgetting that semicolon is a common error. With the following code:

    #include <iostream>

    int main() {
        switch (0) {
            case 0:
                std::cout << "a\n";
                [[fallthrough]]
            case 1:
                std::cout << "b\n";
                break;
        }
    }

G++ (7.1 and the current trunk of 8.0) issues the following warning:

    warning: this statement may fall through [-Wimplicit-fallthrough=]
        std::cout << "a\n";
        ~~~~~~~~~~^~~~~~~~

This is less helpful than it could be. The current Clang trunk provides a
substantially more helpful error message in this case:

    error: fallthrough attribute is only allowed on empty statements
        [[fallthrough]]
          ^
    note: did you forget ';'?
        [[fallthrough]]
                       ^
                       ;

It would be nice to have something similar in G++.