#include <iostream>

namespace foo {
    struct bar {
        int x;
    };
}

int main (void) {
    struct foo::bar *a = new struct foo::bar;
    delete a;
    return 0;
}

并编译：

g++ main.cxx -Wall -Wextra

它给我这个警告：

main.cxx: In function ‘int main()’:
main.cxx:10:39: warning: declaration ‘struct foo::bar’ does not declare anything [enabled by default]

但是,如果我在new关键字后面取出了struct关键字：

#include <iostream>

namespace foo {
    struct bar {
        int x;
    };
}

int main (void) {
    struct foo::bar *a = new foo::bar;
    delete a;
    return 0;
}

并以相同的方式编译,g不输出警告.为什么g输出警告如果我使用struct关键字？