Templated requires clause fails-CodePudding

I have a set of classes roughly defined as follows:

template <typename U>
class Iterable {
    // More code
};

class Container : public Iterable<Element> {
    // More code
};

class Tuple : public Container {
    // More code
};

class List {
    public:

    template <typename I, typename T>
    requires std::is_base_of_v<Iterable<T>,I>
    explicit List(const I& i) {
        for (const T& e : i) elems_.emplace_back(e,true);
    }
    
    // More code
};

Trying to create a List from Tuple as

Tuple t1(1,2,3);
List l1(t1);

gives the following complilation message

/home/felix/git/libraries/cpp_script/tests/test_list.cpp:96:15: error: no matching function for call to ‘cs::List::List(cs::Tuple&)’
   96 |     List l1(t1);
      |               ^
In file included from /home/felix/git/libraries/cpp_script/tests/test_list.cpp:3:
/home/felix/git/libraries/cpp_script/include/list.hpp:72:10: note: candidate: ‘template<class I, class T>  requires  is_base_of_v<cs::Iterable<T>, I> cs::List::List(const I&)’
   72 | explicit List(const I& i) {
      |          ^~~~
/home/felix/git/libraries/cpp_script/include/list.hpp:72:10: note:   template argument deduction/substitution failed:
/home/felix/git/libraries/cpp_script/tests/test_list.cpp:96:15: note:   couldn’t deduce template parameter ‘T’
   96 |     List l1(t1);
      |               ^

I don't understand why the substitution fails. I==Tuple and T==Element should satisfy the require clause just fine.

CodePudding user response：

Your example won't work because there is no way to deduce the type of T.

If you want to constrain I to inherit from base class Iterable<U> for some type U, you might want to do

template <typename U>
class Iterable { };

template<typename T>
concept derived_from_iterable = requires (T& x) {
  []<typename U>(Iterable<U>&){}(x);
};

class List {
 public:
  template<derived_from_iterable I>
  explicit List(const I& i) {
    // More code
  }
};

Demo