Parent template argument deduction in nested class constructor - c++

Question

I am trying to write the "promotion" constructor of a nested class that can deduce the parent class template. It works fine for the parent class, but not in the nested class. Here is a code example.
template <class T>
struct potato {
struct baked {
template <class O>
baked(const typename potato<O>::baked& p)
: something(static_cast<T>(p.something)) {
}
baked() = default;
T something;
};
template <class O>
potato(const potato<O>& p)
: mybaked(p.mybaked) {
}
potato() = default;
baked mybaked;
};
int main(int, char**) {
potato<int> potato1;
potato<short> potato2(potato1);
}
Is this legal?
Various compilers output various errors. Clang has the most readable in my mind. It states :
candidate template ignored: couldn't infer template argument 'O'
https://godbolt.org/z/y_IZiE
So I'm guessing either I've messed up the signature, or this isn't a c++ supported feature.

Answer 1

I don't know of any way to deduce the template argument T for a baked's parent potato<T>. You can know T using decltype(p.something) but that doesn't seem to help solve the problem with calling the constructor. One workaround is to change baked's constructor to take any O and assume it has a something :
struct baked {
template <class O>
baked(const O & p) : something(static_cast<T>(p.something))
{ }
baked() = default;
T something;
};
This will work but it is less type-safe than your original code seems to intend. One workaround for that problem could be to introduce a static_assert that checks that O is actually a potato<U>::baked :
#include <type_traits>
template <class T>
struct potato {
struct baked {
template <class O>
baked(const O & p) : something(static_cast<T>(p.something))
{
using t_parent = potato<decltype(p.something)>;
static_assert(std::is_same<O, typename t_parent::baked>::value, "Not a baked potato!");
}
baked() = default;
T something;
};
template <class O>
potato(const potato<O>& p)
: mybaked(p.mybaked) {
}
potato() = default;
baked mybaked;
};
This should compile fine for the intended usage but fail with "Not a baked potato!" if you try to pass anything else with a something. This would fail :
struct foo {
int something = 0;
};
int main(int, char**) {
foo bar;
potato<int>::baked baz(bar); // Error: Not a baked potato!
}

Answer 2

As state by the compiler O is not deducible from const typename potato<O>::baked& (on left side of ::).
You have several workarounds:
Move baked outside parent and make it template:
// Possibly in namespace details
template <typename T>
struct baked_impl {
template <class O>
baked_impl(const typename baked_impl<O>& p)
: something(static_cast<T>(p.something)) {
}
baked_impl() = default;
T something;
};
template <class T>
struct potato {
using baked = baked_impl<T>;
// ...
};
Add parent info in baked and use SFINAE:
template <class T> struct potato;
// traits for SFINAE
template <class T> struct is_potato : std::false_type {};
template <class T> struct is_potato<potato<T>> : std::true_type {};
template <class T>
struct potato {
using value_type = T;
struct baked {
using parent = potato;
template <class O, std::enable_if_t<is_potato<typename O::parent>::value, int> = 0>
baked(const O& p)
: something(static_cast<typename O::parent::value_type>(p.something)) {
}
baked() = default;
T something;
};
// ...
};