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++.
