[SOLVED] Is there any obvious way to rewrite this code to work around a compiler bug triggering invalid C2244 error?

Issue

The following is valid, standard-conforming code from the GCC source code base. It trips the Visual C++ compilers, triggering error C2244. That is a compiler bug that I already reported, but it's unlikely to get solved soon. Too low of an impact they say. _{Now, I'd be rather miffed if my compiler product choked on a leading open-source compiler codebase, but that's just me.}

Is there some workaround or a way to rewrite this to preserve the API while getting it past VS C++?

Getting the function body inside the class declaration works around the issue, but that's a bit of a stylistic nightmare and has about zero chance of upstreaming. So I was hoping for something less drastic.

Error:

(17): error C2244: 'hash_table::traverse_noresize': unable to match function definition to an existing declaration

Self-contained repro case below - also available on Godbolt. No compile flags needed, and AFAIK it trips all releases of VS 2022.

// minimized excerpt from gcc/hash_table.h

template <class Descriptor> class hash_table
{
  using value_type = typename Descriptor::value_type;
public:
  template <int (*Callback)(value_type *)> void traverse_noresize ();
};

template<class Descriptor>
template<int (*Callback) (typename hash_table<Descriptor>::value_type *)>
void hash_table<Descriptor>::traverse_noresize() {}
// Error C2244 in line above. Apparently, this definition doesn't match the declaration
// in the class body.

struct D { using value_type = int; };
int C(int *slot) { return {}; }

void test()
{
    hash_table<D> ht;
    ht.traverse_noresize<C>();
}

Solution

The problem is that MSVC is resolving the alias hash_table<...>::value_type early, and then failing to resolve it during member function (template) declaration matching. Manually dealiasing at the definition appears to work:

template<typename Descriptor>
template<typename Argument,
     int (*Callback)
     (typename Descriptor::value_type *slot,
//             ^~~~~~~~~~
     Argument argument)>
void hash_table<Descriptor>::traverse_noresize () {}

Demo.

Answered By - ecatmur

Answer Checked By - Mary Flores (WPSolving Volunteer)