warning C4910: '__declspec(dllexport)' and 'extern' are incompatible on an explicit instantiation

[moiwi]

Compiling format.h with FMT_EXPORT defined gives the warning

format.h(998,71): warning C4910: 'fmt::v7::detail::basic_data<void>': '__declspec(dllexport)' and 'extern' are incompatible on an explicit instantiation (compiling source file os.cc)

with Visual Studio 2019.
Why was

#ifndef FMT_EXPORTED

#endif

removed?

[vitaut]

It was removed in #2220 because of clang warnings. It seems that MSVC and clang handle dllexport in incompatible ways.

[vitaut]

We might need both FMT_INSTANTIATION_DECL_API and FMT_INSTANTIATION_DEF_API defined based on the compiler (sigh).

[denchat]

I'm trying to fix this at #2229 .
However I'm not sure that...

#ifndef FMT_EXPORTED
extern template struct
#endif

is also required by msvc.

I don't have msvc, please report back.

[moiwi]

It seems

#ifndef FMT_EXPORTED

#endif

is needed for MSVC. Without I still get ehe same warning

'__declspec(dllexport)' and 'extern' are incompatible on an explicit instantiation

[denchat]

Okey, I wrapped it up.

However, I've been thinking that msvc should never has seen extern template together with dllexport in the 1st place.
The preprocessor is supposed to behave like the previous one on msvc!?
That's odd or maybe I might have missed out something.

Please check again.

[moiwi]

#2229 fixes my problem.
Without "#ifndef FMT_EXPORTED" the preprocessor output is

template <typename T = void> struct __declspec(dllexport) basic_data {
  static const uint64_t powers_of_10_64[];
  //...
  static const uint64_t zero_or_powers_of_10_64[];
};
//...
extern template struct  basic_data<void>;

which results in the mentioned compiler warning.

[vitaut]

Thinking more of it, basic_data doesn't seem to need dllexport at all. At least I was able to build fmt dll with it removed (8bc4550) and the tests work.

[vitaut]

@moiwi, @denchat, does 8bc4550 work for you?

[moiwi]

No, it doesn't work for me (with MSVC 2019).
I still get the line
template <typename T = void> struct FMT_EXTERN_TEMPLATE_API basic_data {
(which gets preprocessed to
template <typename T = void> struct __declspec(dllexport) basic_data {
) colliding with
FMT_EXTERN template struct basic_data<void>;
(which gets preprocessed to
extern template struct basic_data<void>;
)

It does however work if I remove "FMT_EXTERN_TEMPLATE_API" like that:
template <typename T = void> struct /*FMT_EXTERN_TEMPLATE_API*/ basic_data {

[vitaut]

OK, let's go with #2229 then. Thanks for testing.

[denchat]

#2241

The extern template's game is quite a mess between camps!?
Clang: I don't like forward declaration of entity.
MSVC: I don't like extern template on members.

Sigh.