Specialization of class template with auto parameter

Question

The following does not compile with the latest MSVC.

template <auto>
struct S {};

template <int *V>
struct S<V> {};

int main()
{
    S<1>{}; //error C2440: 'specialization': cannot convert from 'int' to 'int *'
}

I thought perhaps an auto parameter is ignored in overload resolution, so I tried the following, which still did not compile to my surprise.

template <auto>
struct S {};

template <int V>
struct S<V> {};

template <int *V>
struct S<V> {};

int main()
{
    S<1>{}; //error C2752: 'S<1>': more than one partial specialization matches the template argument list
}

I think that the second case must be a compiler error. Is this correct?

Answer 1

Hi，

This syntax is valid in the C++ Concepts Technical Specification, but not in C++20. In C++20 concepts, auto is only permitted at the top level in a function parameter type.

For more details I suggest you could refer to the links:
https://stackoverflow.com/questions/60358154/auto-as-a-template-argument-placeholder-for-a-function-parameter
https://stackoverflow.com/questions/38026884/advantages-of-auto-in-template-parameters-in-c17

As far as I'm concerned you could try to use c++17 in the the latest MSVC. (Priperties -> General -> C++ language Standard -> ISO C++17 Standard(std:c++17)).

Best Regards,

Jeanine

---

If the response is helpful, please click "Accept Answer" and upvote it.

Note: Please follow the steps in our documentation to enable e-mail notifications if you want to receive the related email notification for this thread.