[Bug libstdc++/66338] New: std::forward_as_tuple() issue with single argument

[ptomulik at meil dot pw.edu.pl]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=66338

            Bug ID: 66338
           Summary: std::forward_as_tuple() issue with single argument
           Product: gcc
           Version: unknown
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: libstdc++
          Assignee: unassigned at gcc dot gnu.org
          Reporter: ptomulik at meil dot pw.edu.pl
  Target Milestone: ---

Created attachment 35652
  --> https://gcc.gnu.org/bugzilla/attachment.cgi?id=35652&action=edit
Minimal example

Hi,

this example fails to compile (it compiles in clang however):

//////////////////// fail1.cpp ///////////////////////////
#include <tuple>

struct S {
  int i_;

  template<typename T>
  S(T&& i)
    noexcept(noexcept(i_ = i))
  { i_ = i; }

  S() noexcept : i_{0} {};
};

int main()
{
  std::tuple<S&&>(std::forward_as_tuple(S{}));
  return 0;
}
//////////////////// fail1.cpp ///////////////////////////

with the following error message:

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/type_traits:1401:39:   required by substitution of
‘template<class _From1, class _To1, class> static std::true_type
std::__is_convertible_helper<_From, _To, false>::__test(int) [with _From1 =
std::tuple<S&&>; _To1 = S&&; <template-parameter-1-3> = <missing>]’
/usr/include/c++/4.9/type_traits:1410:30:   required from ‘struct
std::__is_convertible_helper<std::tuple<S&&>, S&&, false>’
/usr/include/c++/4.9/type_traits:1416:12:   required from ‘struct
std::is_convertible<std::tuple<S&&>, S&&>’
/usr/include/c++/4.9/type_traits:129:12:   required from ‘struct
std::__and_<std::is_convertible<std::tuple<S&&>, S&&> >’
/usr/include/c++/4.9/tuple:402:40:   required by substitution of
‘template<class ... _UElements, class> constexpr std::tuple<
<template-parameter-1-1> >::tuple(_UElements&& ...) [with _UElements =
{std::tuple<S&&>}; <template-parameter-1-2> = <missing>]’
test.cpp:16:45:   required from here
test.cpp:8:26: error: cannot convert ‘std::tuple<S&&>’ to ‘int’ in assignment
     noexcept(noexcept(i_ = i))



If I remove the noexcept(noexcept(i_ = i)) or invoke forward_as_tuple() with
two arguments, then it compiles without a problem.


///////////////////////////// ok1.cpp ////////////////////////////
#include <tuple>

struct S {
  int i_;

  template<typename T>
  S(T&& i)
//    noexcept(noexcept(i_ = i))
  { i_ = i; }

  S() noexcept : i_{0} {};
};

int main()
{
  std::tuple<S&&>(std::forward_as_tuple(S{}));
  return 0;
}
///////////////////////////// ok1.cpp ////////////////////////////



///////////////////////////// ok2.cpp ////////////////////////////
#include <tuple>

struct S {
  int i_;

  template<typename T>
  S(T&& i)
    noexcept(noexcept(i_ = i))
  { i_ = i; }

  S() noexcept : i_{0} {};
};

int main()
{
  std::tuple<S&&,S&&>(std::forward_as_tuple(S{}, S{}));
  return 0;
}
///////////////////////////// ok2.cpp ////////////////////////////

Attached an example which fails to compile (fail1.cpp).