C++ local variables in return argument list (as seen on CodeFights)

I've seen this form of syntax on CodeFights.com for C++ functions:

int i,j,w,myFunction(auto s, auto v) {
  // here, i, j,and w are local variable, apparently initialized to 0 
  // all work done inside this function is typical, legal C++
  return [some int value];
}

I've been searching the web to identify this syntax but I cannot. Is this legal C++ or a syntax particular to CodeFights? Can someone give me the name of this kind of initialization so that I can look it up?