$ ../cpla.sh  p178-2
$ clang++ p178-2.cpp -std=c++03 -Wall
p178-2.cpp:12:1: error: declaration of anonymous struct must be a definition
struct [[nodiscard]] error_info { /*...*/ };
^
p178-2.cpp:12:1: warning: declaration does not declare anything [-Wmissing-declarations]
p178-2.cpp:14:1: error: unknown type name 'error_info'
error_info ei;
^
p178-2.cpp:15:1: error: unknown type name 'error_info'
error_info & e=ei;
^
p178-2.cpp:17:1: error: unknown type name 'error_info'
error_info enable_missile_safety_mode(){return ei;}
^
p178-2.cpp:24:1: error: unknown type name 'error_info'
error_info &foo(){return e;}
^
1 warning and 5 errors generated.
$ clang++ p199.cpp -std=c++2a -Wall
p178-2.cpp:20:1: warning: ignoring return value of function declared with 'nodiscard' attribute
      [-Wunused-result]
enable_missile_safety_mode(); // warning encouraged
^~~~~~~~~~~~~~~~~~~~~~~~~~
1 warning generated.
f(){foo();}
testmissiles(){enable_missile_safety_mode();launch_missiles();}
C++N4741(97)10.6.7 Nodiscard attribute [dcl.attr.nodiscard]p178-2.cpp

$ g++-8 p199.cpp -std=c++03  -Wall
p199.cpp:12:20: warning: c++11 attributes only available with -std=c++11 or -std=gnu++11
 struct [[nodiscard]] error_info { /*...*/ };
                    ^
p178-2.cpp: In function 'void test_missiles()':
p199.cpp:20:28: warning: ignoring returned value of type 'error_info', declared with attribute nodiscard [-Wunused-result]
 enable_missile_safety_mode(); // warning encouraged
                            ^
p199.cpp:17:12: note: in call to 'error_info enable_missile_safety_mode()', declared here
 error_info enable_missile_safety_mode(){return ei;}
            ^~~~~~~~~~~~~~~~~~~~~~~~~~
p178-2.cpp:12:22: note: 'error_info' declared here
 struct [[nodiscard]] error_info { /*...*/ };
                      ^~~~~~~~~~
f(){foo();}
testmissiles(){enable_missile_safety_mode();launch_missiles();}
C++N4741(97)10.6.7 Nodiscard attribute [dcl.attr.nodiscard]p178-2.cpp

$ g++-8 p178-2.cpp -std=c++2a  -Wall
p178-2.cpp: In function 'void test_missiles()':
p178-2.cpp:20:28: warning: ignoring returned value of type 'error_info', declared with attribute nodiscard [-Wunused-result]
 enable_missile_safety_mode(); // warning encouraged
                            ^
p178-2.cpp:17:12: note: in call to 'error_info enable_missile_safety_mode()', declared here
 error_info enable_missile_safety_mode(){return ei;}
            ^~~~~~~~~~~~~~~~~~~~~~~~~~
p178-2.cpp:12:22: note: 'error_info' declared here
 struct [[nodiscard]] error_info { /*...*/ };
                      ^~~~~~~~~~
f(){foo();}
testmissiles(){enable_missile_safety_mode();launch_missiles();}
C++N4741(97)10.6.7 Nodiscard attribute [dcl.attr.nodiscard]p178-2.cpp