What code fragments contain errors (compilation, e

What code fragments contain errors (compilation, execution, logical)?

struct A {    // 1
  int &a;
  A(int b) { a = b; }
};

struct A {    // 2
    int &a; 
    A(int b) : a(b) { } 
};

struct A {    // 3
    A &a;
    A() : a(*this) { } 
};

struct A {    // 4
    A a; 
    A() : a(*this) { } 
};

Explanation

(1) - error, because reference a can be initialized only in the initialization list.
(2) - an error because the reference a refers to the local variable b passed to the constructor as an argument. When exiting the constructor, the b variable will be destroyed and the value a becomes undefined.
(3) - no error. Here the reference a is equivalent to *this.
(4) - error. A is incomplete type while defining A a;