[Bug libstdc++/100667] [11/12 Regression] std::tuple<A&&> cannot be constructed from A&&, if A not defined (only forward declared)

["ostash at ostash dot kiev.ua" <gcc-bugzilla@gcc.gnu.org>]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=100667

Viktor Ostashevskyi <ostash at ostash dot kiev.ua> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
                 CC|                            |ostash at ostash dot kiev.ua

--- Comment #1 from Viktor Ostashevskyi <ostash at ostash dot kiev.ua> ---
Hello,

I have another example, but probably related:


#include <tuple>
#include <string>

class Foo{};

std::tuple<std::string, Foo> bar()
{
    return { {}, Foo{}};
}


Fails starting from GCC 11.1 in C++11/14/17/20 modes with the following error:

<source>: In function 'std::tuple<std::__cxx11::basic_string<char,
std::char_traits<char>, std::allocator<char> >, Foo> bar()':
<source>:8:23: error: conversion from '<brace-enclosed initializer list>' to
'std::tuple<std::__cxx11::basic_string<char, std::char_traits<char>,
std::allocator<char> >, Foo>' is ambiguous
    8 |     return { {}, Foo{}};
      |                       ^
In file included from <source>:1:
/opt/compiler-explorer/gcc-11.1.0/include/c++/11.1.0/tuple:1144:9: note:
candidate: 'std::tuple<_T1, _T2>::tuple(std::allocator_arg_t, const _Alloc&)
[with _Alloc = Foo; typename
std::enable_if<std::_TupleConstraints<std::is_object<_Alloc>::value, _T1,
_T2>::__is_implicitly_default_constructible(), bool>::type <anonymous> = true;
_T1 = std::__cxx11::basic_string<char>; _T2 = Foo]'
 1144 |         tuple(allocator_arg_t __tag, const _Alloc& __a)
      |         ^~~~~
/opt/compiler-explorer/gcc-11.1.0/include/c++/11.1.0/tuple:1052:9: note:
candidate: 'constexpr std::tuple<_T1, _T2>::tuple(const _T1&, const _T2&) [with
bool _Dummy = true; typename std::enable_if<std::_TupleConstraints<_Dummy, _T1,
_T2>::__is_implicitly_constructible<const _T1&, const _T2&>(), bool>::type
<anonymous> = true; _T1 = std::__cxx11::basic_string<char>; _T2 = Foo]'
 1052 |         tuple(const _T1& __a1, const _T2& __a2)
      |