C++, why is forward declaration not required in structs/classes

Question

In C++, why does the following compile:

struct test {
    void foo() { this->bar++; }
    int bar;
};

But not the following:

Answer 1

In case of a structure, scope of the member variable is available within the structure. So if you define a function within a structure that variable will be available within the function.

But that is the not the case in second scenario.