c++ - How to use C++20 concepts to check some constraints in the template itself

Question

Sorry for the ambiguous title, I couldn't find the right words to phrase it. I'm trying to force a class to define some functions/operators to be correct. for example ForwardIterator is required to have some operators otherwise I should get a compiler error. I have asked the question here and I was given a method with inheritance, it works perfectly but C++20 concepts seem to be more intuitive and give better errors. The initial idea is:

template <typename T> concept is_ForwardIterator= requires(T x, T y)
{
    x == y;
};
template <typename T>
requires  is_ForwardIterator<T>
struct ForwardIterator {
};

Then whenever I want to implement a forward iterator I inherit from ForwardIterator:

struct MyCustomIterator
   : public ForwardIterator <MyCustomIterator> 
{
  bool operator ==(const MyCustomIterator& other) const;
};

But the compiler keeps complaining that MyCustomIterator doesn't satisfy the requirement:

error: constraints not satisfied for class template 'ForwardIterator' [with T = MyCustomIterator]
note: because 'MyCustomIterator' does not satisfy 'is_ForwardIterator'
template <typename T> requires is_ForwardIterator<T> struct ForwardIterator {
                               ^
note: because 'x == y' would be invalid: invalid operands to binary expression ('MyCustomIterator' and 'MyCustomIterator')
        x == y;

Answer 1

In CRTP, derived class in incomplete, so checking its property is not possible.

One way to check interface with traits/concept is simply static_assert after the class:

template <typename T> concept is_ForwardIterator= requires(T x, T y)
{
    x == y;
    // ...
};

struct MyCustomIterator
{
    bool operator ==(const MyCustomIterator& other) const;
    // ...
};
static_assert(is_ForwardIterator<MyCustomIterator>);