C++11 Default member variable value + brace-enclosed initializer list

Question

The code below works:

   #include <iostream>                                                             

   using namespace std;                                                            

   struct Formula {                                                                
     int x;                                                                   
   };                                                                              

   int main() {                                                                    
    Formula f = {1};                                                              
    return 0;                                                                     
  } 

However, after adding the default value for the member variable, it fails:

   #include <iostream>                                                             

   using namespace std;                                                            

   struct Formula {                                                                
     int x = 10; // Initialize x with a default value 10.                                                            
   };                                                                              

   int main() {                                                                    
    Formula f = {1}; // this line no longer works.                                                           
    return 0;                                                                     
  } 

The error is below:

g++ -std=gnu++11 exp07.cc 
exp07.cc: In function ‘int main()’:
exp07.cc:10:17: error: could not convert ‘{1}’ from ‘<brace-enclosed initializer list>’ to ‘Formula’
   Formula f = {1};