C++: Instantiation & Variable Declaration

Problem

Call an overloaded operator= on a temporary instance of a class.

Given a class A:

struct A {
   A(int arg) { }
   A& operator=(int num) { return *this; }
};

The following code will not compile with gcc-4.6.1 due to a "conflicting declaration" error:

int a;
A(a) = 4; // error: conflicting declaration

The compiler thinks that with A(a) we are trying to declare an instance of A named a.

Workaround

Wrapping the instantiation with round brackets will fix the problem.

int a;
(A(a)) = 4;

Strangely, when the operator is not the assignment one, the code compiles.

struct A {
   A(int arg) { }
   A& operator+=(int num) { return *this; }
};
...
int a;
A(a) += 4;