Here is minimal example:
#include <cstddef>
struct base_pod
{
};
template<typename T, std::size_t N>
struct der_pod : public base_pod
{
T k[N];
};
template<typename T, typename... U>
der_pod(T, U...)
->der_pod<std::enable_if_t<(std::is_same_v<T, U> and ...), T>, 1 + sizeof...(U)>;
int main()
{
der_pod dp {{}, {3, 3} };
}
prog.cc:16:9: error: no viable constructor or deduction guide for deduction of template arguments of 'der_pod'
der_pod dp {{}, {3, 3} };
^
prog.cc:11:2: note: candidate template ignored: couldn't infer template argument 'T'
der_pod(T, U...)
^
prog.cc:6:8: note: candidate function template not viable: requires 0 arguments, but 2 were provided
struct der_pod : public base_pod
^
prog.cc:6:8: note: candidate function template not viable: requires 1 argument, but 2 were provided
1 error generated.
I have to add explicit template argumentder_pod<int, 2> to pass compilation.
But deduction guide works fine when it has no base class:
template<typename T, std::size_t N>
struct der_pod
{
T k[N];
};
template<typename T, typename... U>
der_pod(T, U...)
->der_pod<std::enable_if_t<(std::is_same_v<T, U> and ...), T>, 1 + sizeof...(U)>;
int main()
{
der_pod dp {3, 3};
}
{}, {3, 3} have no type and don't allow deduction of T and U.
Intead, you might use:
template <typename T, std::size_t N>
der_pod(base_pod, const T (&)[N]) -> der_pod<T, N>;