混合类和结构

I'm well aware of the difference between class and struct, however I'm struggling to authoritatively say if this is well defined:

// declare foo (struct)
struct foo;

// define foo (class)
class foo {
};

// instance of foo, claiming to be a struct again! Well defined?
struct foo bar;

// mixing class and struct like this upsets at least one compiler (names are mangled differently)
const foo& test() {
   return bar;
}

int main() {
   test();
   return 0;
}

如果这是未定义的行为，有人可以指出我权威(即 ISO 中的章节和诗句)参考的方向吗?