https://gcc.gnu.org/bugzilla/show_bug.cgi?id=101906
Bug ID: 101906
Summary: Constant evaluation failure in concepts
Product: gcc
Version: 12.0
Status: UNCONFIRMED
Severity: normal
Priority: P3
Component: c++
Assignee: unassigned at gcc dot gnu.org
Reporter: de34 at live dot cn
Target Milestone: ---
Following code fails to compile when the macro MAYBE_CWG1581 is not defined.
#include <bit>
#include <cstddef>
namespace test {
template<int> using voidify = void;
template<std::size_t N>
struct obj_rep { unsigned char value[N]; };
template<class T>
concept constant_value_initializable =
requires { typename voidify<(std::bit_cast<obj_rep<sizeof(T)>>(T()), 0)>;
};
}
struct foo {
int x;
};
struct bar {
int x = -1;
};
#ifdef MAYBE_CWG1581
using my_void =
test::voidify<(std::bit_cast<test::obj_rep<sizeof(bar)>>(bar()), 0)>;
#endif
static_assert(test::constant_value_initializable<int>);
static_assert(test::constant_value_initializable<foo>);
static_assert(test::constant_value_initializable<bar>); // It's buggy
static_assert(!test::constant_value_initializable<int*>);
// Correct: std::bit_cast doesn't perform constant evaluation when involving
pointers
int main()
{
}
The godbolt link: https://gcc.godbolt.org/z/6dscsaGTM
It seems that currently gcc fails to generate the definition of
std::bit_cast<SomeType> when determining if the concept is satisfied, while it
can generate such definition when a type alias declaration is encountered.
Maybe related to WG21 P0859R0/CWG1581.
