Type deduction of a recursive function that returns auto

Question | Jul 9, 2019 | nextptr 

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A C++11 compiler can deduce the return type of a lambda expression that has no explicit return type (or has auto return type):

 /* Define and invoke a lambda that returns a double */
 auto lret = [] () { return 10.5; }();

 // 'lret' is deduced as double 
 std::cout << typeid(lret).name() << "\n"; // logs 'd' or 'double'

The C++14 standard extended that feature to regular functions:

// Return type is deduced to int 
auto func() { return 10; }

auto fret = func();

// 'fret' is deduced as int 
std::cout << typeid(fret).name() << "\n"; // logs 'i' or 'int'

However, there are some conditions when the compiler cannot deduce the return type of a function. One of those restrictions is on the implementation of recursive functions. These are 2 equivalent implementations of the recursive factorial function:


auto factorial(uint32_t n) {
    return n*factorial(n-1);
  return 1U;


auto factorial(uint32_t n) {
  if(n == 0)
    return 1U;
  return n*factorial(n-1); 

Select which of A and/or B can be successfully compiled (Check Explanations to learn about correct answer):