C ++ 17中名称空间的属性“不建议使用”

Question

I have created C++ program to experiment of deprecated keyword of C++17.

It's working fine in clang(3.8.0) , But, g++(5.4.0) gives an error.

source_file.cpp:9:11: error: expected identifier before ‘[’ token
 namespace [[ deprecated ]] bar
           ^
source_file.cpp:9:28: error: ‘bar’ does not name a type
 namespace [[ deprecated ]] bar
                            ^
source_file.cpp: In function ‘int main()’:
source_file.cpp:16:5: error: ‘bar’ has not been declared
     bar::var = 10;

Why?

#include <iostream>
using namespace std;

namespace foo  
{ 
  int var; 
}

namespace [[ deprecated ]] bar
{ 
  int var; 
}

int main() 
{
    bar::var = 10;
    return 0;
}

Answer 1

The clue is in the error message: "expected identifier before [ token". The identifier is bar , and it has to appear before [ . So:

namespace bar [[deprecated]]
{ } 

Tested with GCC6.3