https://gcc.gnu.org/bugzilla/show_bug.cgi?id=102535
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-initializations
(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 >= 201902L
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 and B
(both treated as xvalues according to [meta.unary.prop]/8) only calls trivial
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 true
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
