https://gcc.gnu.org/bugzilla/show_bug.cgi?id=66338
--- Comment #2 from Pawel Tomulik <ptomulik at meil dot pw.edu.pl> ---
I found this to be related to std::tuple constructors, especially this one:
template<typename... _UElements, typename = typename
enable_if<__and_<is_convertible<_UElements,
_Elements>...>::value>::type>
explicit
constexpr tuple(_UElements&&... __elements)
: _Inherited(std::forward<_UElements>(__elements)...) { }
in some circumstances it gets precedence over the copy constructor.. for
example:
/////////////////////////////////////////////////////////////////////////////
#include <tuple>
struct S {
int i_;
template<typename T>
S(T&& i)
{ i_ = i; }
S() noexcept : i_{0} {};
};
int main()
{
S const s{};
std::tuple<S> t1{s};
std::tuple<S>{t1}; // well... tuple<S> looks like being "convertible" to S
return 0;
}
////////////////////////////////////////////////////////////////////////////
ptomulik@barakus:$ g++ -Wall -Wextra -Werror -pedantic -g -O0 -std=c++11 -o
test-gcc test.cpp
test.cpp: In instantiation of ‘S::S(T&&) [with T = std::tuple<S>&]’:
/usr/include/c++/4.9/tuple:142:42: required from ‘constexpr
std::_Head_base<_Idx, _Head, false>::_Head_base(_UHead&&) [with _UHead =
std::tuple<S>&; long unsigned int _Idx = 0ul; _Head = S]’
/usr/include/c++/4.9/tuple:264:38: required from ‘constexpr
std::_Tuple_impl<_Idx, _Head, _Tail ...>::_Tuple_impl(_UHead&&, _UTail&& ...)
[with _UHead = std::tuple<S>&; _UTail = {}; <template-parameter-2-3> = void;
long unsigned int _Idx = 0ul; _Head = S; _Tail = {}]’
/usr/include/c++/4.9/tuple:407:54: required from ‘constexpr std::tuple<
<template-parameter-1-1> >::tuple(_UElements&& ...) [with _UElements =
{std::tuple<S>&}; <template-parameter-2-2> = void; _Elements = {S}]’
test.cpp:17:19: required from here
test.cpp:8:8: error: cannot convert ‘std::tuple<S>’ to ‘int’ in assignment
{ i_ = i; }
Is it intended?
Anyway, adding explicit to S(T&&) solves the problem...
