C++14 lambda with init capture does not generate valid code

[sweenish]

This is the code I wanted to see expanded; it is valid C++ and compiles fine on my Catalina MBP.

#include <iostream>
#include <memory>

int main()
{
    auto x = std::make_unique<int>(42);
    auto lambda = [x = std::move(x)]() { std::cout << *x << '\n'; };
}

Here is the generated code:

#include <iostream>
#include <memory>

int main()
{
  std::unique_ptr<int, std::default_delete<int> > x = std::unique_ptr<int, std::default_delete<int> >(std::make_unique<int>(42));
    
  class __lambda_7_19
  {
    public: 
    inline void operator()() const
    {
      std::operator<<(std::cout.operator<<(x.operator*()), '\n');
    }
    
    private: 
    std::unique_ptr<int, std::default_delete<int> > x;
    public: 
    // inline __lambda_7_19(const __lambda_7_19 &) = delete;
    // inline __lambda_7_19(__lambda_7_19 &&) noexcept = default;
    public: __lambda_7_19(std::unique_ptr<int, std::default_delete<int> > _x)
    : x{_x}  // *** ERROR HERE ON _x ***
    {}
    
  };
  
  __lambda_7_19 lambda = __lambda_7_19(__lambda_7_19{std::unique_ptr<int, std::default_delete<int> >(std::move(x))});
}

And this is the error message I get from the generated code:

std::__1::unique_ptr<int, std::__1::default_delete<int>> _x
function "std::__1::unique_ptr<_Tp, _Dp>::unique_ptr(const std::__1::unique_ptr<int, std::__1::default_delete<int>> &) throw() [with _Tp=int, _Dp=std::__1::default_delete<int>]" (declared implicitly) cannot be referenced -- it is a deleted function

The generated constructor attempts to make a copy of a type I am attempting to init capture in my lambda. This type also happens to be non-copyable. For reference, if it was a vector instead of unique_ptr, the generated code works because it can be [and is] copied.

I am experiencing this error using the website cppinsights.io, and using C++14 with the option "show all implicit casts" checked.

[andreasfertig]

Hello @sweenish,

thank you for reporting this. At this point there is some lifting involved to generate the code of the lambda. I check whether it is possible to handle this case correctly.

Andreas