Why can't std::vector<std::jthread>> can't be initialized via an std::initializer_list?

Question

I have an 2 example codes where I have a vector of jthreads:

#include <thread>
#include <vector>
#include <iostream>

int main() {
    std::vector<std::jthread> threads;
    
    threads.emplace_back([] {
        std::cout << "New thread via .emplace_back()" << std::endl;
    });
}

Here, everything works fine as expected, but I get an error when I convert this code into something like below:

#include <thread>
#include <vector>
#include <iostream>

int main() {
    std::vector<std::jthread> threads {
        {[] {
            std::cout << "New thread during initialization" << std::endl;
        }}
    };
}

What is wrong with the second case? From what I see neither gcc nor clang supports it.

• Is there an undefined behavior? std::vector constructor #5 specifies it might be due to that the element is not EmplaceConstructible, but in that case wouldn't the first one also give out an error?
• Is there something related to constexpr?
• Maybe something else? I see the following in the compilation error:
  • error: use of deleted function 'std::jthread::jthread(const std::jthread&)'
  • Why is it trying to use the copy constructor in the first place? Isn't the main reason for such an initialization to prevent unnecessary copy constructions?
  • I've found this question Why would std::initializer_list copy items?, but I can't find the reference its answer mentions.

Answer 1

First of all, std::jthread has an explicit constructor. Thus:

auto lambda = [] {
        std::cout << "New thread during initialization" << std::endl;
};
std::jthread t1 = lambda; // impossible
std::jthread t2 = std::jthread(lambda); // possible

The second thing. To use the std::initializer_list<> like so, the type must be copyable (std::jthread isn't). I think the compiler would probably optimize the copy since the initializer_list is a temporary value. But sadly it is required. Technically, initializer_list has an array which holds the data.

std::vector<std::jthread> threads = {std::jthread(lambda)}; 
// error: use of deleted function 'std::jthread::jthread(const std::jthread&)'