Compilation error with MSVC 19

[Thibaut Cuvelier]

On Tue, 29 Dec 2020 at 23:50, Yuriy Skalko <yuriy.skalko at gmail.com> wrote:

> > I just tried to recompile LyX on MSVC 19, this is the error I get:
> >
> >
> > [ 40%] Building CXX object
> > src/frontends/qt/CMakeFiles/frontend_qt.dir/GuiApplication.cpp.obj
> > GuiApplication.cpp
> > D:\LyX\lyx-unstable\3rdparty\nod\nod.hpp(272): error C2653: 'result_of':
> is
> > not a class or namespace name
> > D:\LyX\lyx-unstable\3rdparty\nod\nod.hpp(323): note: see reference to
> class
> > template instantiation 'nod::signal_accumulator<S,T,F,A...>' being
> compiled
> > D:\LyX\lyx-unstable\3rdparty\nod\nod.hpp(272): error C2146: syntax error:
> > missing ';' before identifier 'type'
> > D:\LyX\lyx-unstable\3rdparty\nod\nod.hpp(272): error C4430: missing type
> > specifier - int assumed. Note: C++ does not support default-int
> >
> > It looks like it's due to bda45704005d6b328e18457a07d05e56883c2874, with
> > the replacement of boost::signals2 (a heavy library) by nod (an
> > unmaintained library). It's about things that are getting deleted in the
> > latest versions of C++, meaning that it will probably bite other
> compilers
> > at some point in the future.
> >
> > As the author of the library did not answer to PRs in the last two years,
> > I'm not sure it's possible to upstream changes to it… What would be best
> in
> > this case?
>
> I assume that you are compiling LyX in C++20 mode since `result_of` is
> only deprecated in C++17. Does the attached patch resolves this issue?
> (It is strange, but seems like result_of still exists in C++20 mode of
> GCC...)
>
> The nod library is pretty small and I suppose the only maintenance will
> be due to backward incompatibilities of newer C++ standards.
>

I did not specify anything in particular, I let CMake deal with the
details, actually. Forcing the C++20 mode did not work, with a
set(CMAKE_CXX_STANDARD 20) in the CMakeFile.

Unfortunately, your patch did not work, neither the corresponding code from
Abseil (
https://github.com/abseil/abseil-cpp/commit/e19260fd7dbef881492fd73891e0be5bd4a09b95
).

Based on https://github.com/ReactiveX/RxCpp/issues/530, I tried with the
_HAS_CXX17 macro, and it worked. I'm attaching the relevant patch, but I'm
not sure it will work on other compilers.
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