Possible compiler bug in MSVC

Question

The following code compiles with gcc and clang (and many other C++11 compilers)

#include <stdint.h>

typedef int datatype;

template <typename T>
struct to_datatype {};

template <>
struct to_datatype<int16_t> {
  static constexpr datatype value = 1;
};

template <typename T>
class data {
 public:
  data(datatype dt = to_datatype<T>::value) {}
};

int main() {
  data<char> d{to_datatype<int16_t>::value};
}

when compile with (almost) latest MSVC

> cl .\test.cpp /std:c++latest /permissive-
Microsoft (R) C/C++ Optimizing Compiler Version 19.24.28314 for x64
Copyright (C) Microsoft Corporation.  All rights reserved.

test.cpp
.\test.cpp(16): error C2039: 'value': is not a member of 'to_datatype<T>'
        with
        [
            T=char
        ]
.\test.cpp(16): note: see declaration of 'to_datatype<T>'
        with
        [
            T=char
        ]
.\test.cpp(20): note: see reference to class template instantiation 'data<char>' being compiled

Is this a bug of MSVC? If yes, which term in C++ standard best describe it?

If you replace part of the code with

template <typename T>
class data {
 public:
  data(datatype dt) {}
  data() : data(to_datatype<T>::value) {}
};

it compiles smoothly anyway.

Answer 1

I would say MSVC is wrong not to accept the code.

According to [dcl.fct.default]/5 of the C++17 standard final draft, name lookup in default arguments of a member function of a class template is done according to the rules in [temp.inst].

According to [temp.inst]/2 implicit instantiation of a class template does not cause instantiation of default arguments of member functions and according to [temp.inst]/4 a default argument for a member function of a (non-explicit specialization of a) class template is instantiated when it is used by a call.

There is no call using the default argument to_datatype<T>::value in your code and so it should not be instantiated. Therefore there shouldn't be an error about lookup of value in to_datatype<char> failing.

(The relevant sections in the C++11 standard final draft have equivalent wording, except for numbering, see [decl.fct.default]/5, [temp.inst]/1 and [temp.inst]/3 instead.)