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From: "de34 at live dot cn" <gcc-bugzilla@gcc.gnu.org>
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Subject: [Bug c++/102535] New: __is_trivially_constructible rejects some
 trivial cases in aggregate initializations
Date: Thu, 30 Sep 2021 02:45:01 +0000
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https://gcc.gnu.org/bugzilla/show_bug.cgi?id=3D102535

            Bug ID: 102535
           Summary: __is_trivially_constructible rejects some trivial
                    cases in aggregate initializations
           Product: gcc
           Version: 10.1.0
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: c++
          Assignee: unassigned at gcc dot gnu.org
          Reporter: de34 at live dot cn
  Target Milestone: ---

It seems that gcc's __is_trivially_constructible (the underlying mechanism =
of
std::is_trivially_constructible in libstdc++) gives inconsistent answers to
whether aggregate initializations performed by direct-non-list-initializati=
ons
(available since C++20) affect triviality of operations.

#include <type_traits>
#include <iostream>

struct A { int x; };
struct B { float y; };
struct C { A a; B b; };

int main()
{
#if __cpp_aggregate_paren_init >=3D 201902L=20
    std::cout
        << std::is_constructible_v<C, A> << '\n'               // 1
        << std::is_constructible_v<C, A, B> << '\n'            // 1
        << !std::is_constructible_v<C, B> << '\n'              // 1
        << std::is_trivially_constructible_v<C, A> << '\n'     // 1
        << std::is_trivially_constructible_v<C, A, B> << '\n'; // 0, seems
buggy
#endif
}

According to the definitions, initialization of a C variable from A or A an=
d B
(both treated as xvalues according to [meta.unary.prop]/8) only calls trivi=
al
move constructors. It seems unreasonable that
std::is_trivially_constructible_v<C, A> is true but
std::is_trivially_constructible_v<C, A, B> is false, they should be both tr=
ue
in C++20.

This bug has been existing since gcc 10.1.0, and is not fixed in 12.0.0
20210721:
https://wandbox.org/permlink/L0ZLipQZLsCOqYcT
https://wandbox.org/permlink/lBwowJpeFshRC03z=