VC++ 15 calls the wrong copy constructor for lambda capture?

Question

Consider the following program:

#include <iostream>
struct X {
  X () = default;
  X (X &) { std::cout << "non-const called" << std::endl; }
  X (X const &) { std::cout << "const called" << std::endl; }
  int i () const { return 7; }
};

auto f () {
  X x;
  auto lambda = [=]() { return x.i(); };
  return lambda;
}

int main()
{
  auto lambda = f();
  std::cout << lambda () << std::endl;
  return 0;
}

With VC++15, I get the output

const called
const called
7

With Clang 3.9, I get

non-const called
7

Which compiler is correct here?

Bronchiectasis

This may be of your interest. To be straight I don't think your question is a duplicate :) – Huoh

It's worth noting that with optimizations enabled, VC also applies RVO resulting in only one call to X(X const&). – Chill

@skypjack: Are you sure lambdas construct a const member? ideone.com/RQ7OjC I think the data members are not necessarily const, only operator () is. – Bronchiectasis

@Bronchiectasis Fair enough. I'm correcting the answer too. Good catch. – Romanfleuve

@Chill still the call should be to X(X &), right? – Romanfleuve

@Romanfleuve As far as I know, right. – Chill

Answer 1

I would say that clang is right.
The most suited constructor is called only once when the lambda captures x and the constructor for the returned value is optimized out.
That's why you obtain only one non-const called.

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See here and here for further details about copy-elision and RVO.

Romanfleuve