In C++11, constexpr function bodies could only contain a very limited set of syntaxes, including (but not limited to): typedefs, usings, and a single return statement. In C++14, the set of allowable syntaxes expands greatly to include the most common syntax such as if statements, multiple returns, loops, etc.

constexpr int factorial(int n) {
  if (n <= 1) {
    return 1;
  } else {
    return n * factorial(n - 1);
  }
}
factorial(5); // == 120