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#include <cstdint>
#include <type_traits>
 
struct chameleon
{
    template<typename T>
    constexpr operator T() const
    {
        return T{};
    }
 
    constexpr chameleon() = default;
};
 
template<std::size_t I>
struct placeholder : public chameleon
{
    using chameleon::chameleon;
};
 
constexpr const placeholder<0> _1;
 
using t = std::integral_constant<int,_1>;
 
int main()
{
 
 
    return 0;
}
 

prog.cpp:23:38: error: the value of ‘_1’ is not usable in a constant expression
 using t = std::integral_constant<int,_1>;
                                      ^
prog.cpp:21:32: note: ‘_1’ used in its own initializer
 constexpr const placeholder<0> _1;
                                ^

Standard output is empty