Why is GCC warning me about a fallthrough even when I use [[fallthrough]]?

Question

In the following piece of code, I use the standard [[fallthrough]] attribute from C++1z to document that a fallthrough is desired:

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

With GCC 7.1, the code compiles without an error. However, the compiler still warns me about a fallthrough:

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

Why?

Answer 1

You are missing a semicolon after the attribute:

case 0:     std::cout << "a\n";     [[fallthrough]];     //             ^ case 1: 

The [[fallthrough]] attribute is to be applied to an empty statement (see P0188R1). The current Clang trunk gives a helpful error in this case:

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

Update: Cody Gray reported this issue to the GCC team.