[Bug c++/81952] New: copy elision used when constructor needs to be called

["barry.revzin at gmail dot com" <gcc-bugzilla at gcc dot gnu dot org>]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=81952

            Bug ID: 81952
           Summary: copy elision used when constructor needs to be called
           Product: gcc
           Version: 7.2.0
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: c++
          Assignee: unassigned at gcc dot gnu.org
          Reporter: barry.revzin at gmail dot com
  Target Milestone: ---

>From StackOverflow (https://stackoverflow.com/q/45843428/2069064), a reduced
example:

#include <iostream>

template <class T>
struct opt {
    opt() { }
    opt(opt const& ) { std::cout << "1a\n"; }
    opt(opt&& ) { std::cout << "1b\n"; }

    template <class U>
    opt(U&& ) { std::cout << "2\n"; }
};

struct foo 
{
    explicit operator opt<int>() { std::cout << "3\n"; return {}; }
};

int main()
{
    opt<int> o(foo{});
}

This is direct-initialization of an object not of the same type, so we need to
go through the constructors, the best of which is the template so this program
should print 2. But instead, gcc elides this and just invokes the opt<int>
conversion function and prints 3 (but not 1b, just 3).