Re: isnan function conflicts with C++ standard library declarations

[Martin Storsjö]

Hi,

On Wed, 28 Aug 2019, Bruno Haible wrote:

> Hi,
>
> Martin Storsjö wrote:
> > When the isnan function is enabled in gnulib, the added bits in math.h
> > break if included in C++ translation units.
> >
> > A minimal reproducion example is available at
> > https://martin.st/temp/gnulib-isnan-repro-0.0.0.tar.gz (preconfigured and
> > directly buildable) and
> > https://martin.st/temp/gnulib-isnan-repro-source.tar.gz (original source,
> > requires running gnulib-tool and autoreconf).
> >
> > The error manifests both with GCC/libstdc++ and Clang/libc++, with error
> > messages like these:
> >
> > In file included from myprog.cpp:2:0:
> > lib/math.h: In function ‘int isnan(float)’:
> > lib/math.h:2829:1: error: ‘int isnan(float)’ conflicts with a previous 
> > declaration
> >   _GL_MATH_CXX_REAL_FLOATING_DECL_2 (isnan)
> >   ^
> > In file included from /usr/include/c++/7/math.h:36:0,
> >                   from lib/math.h:27,
> >                   from myprog.cpp:2:
> > /usr/include/c++/7/cmath:618:3: note: previous declaration ‘constexpr bool 
> > std::isnan(float)’
> >     isnan(float __x)
> >     ^~~~~
>
> I reproduce the issue on mingw - thanks for a very nicely made
> reproduction example -, and have applied the fix below.

Thanks for the very quick fix!

> > In file included from myprog.cpp:2:
> > lib/math.h:2829:36: error: 'isnan' is missing exception specification 'throw()'
> > _GL_MATH_CXX_REAL_FLOATING_DECL_2 (isnan)
> >                                     ^
> > /Applications/Xcode.app/Contents/Developer/Toolchains/XcodeDefault.xctoolchain/usr/include/c++/v1/math.h:533:1:
> >  note:
> >        previous declaration is here
> > isnan(float __lcpp_x) _NOEXCEPT { return __libcpp_isnan(__lcpp_x); }
> > ^
>
> Probably I should test this also on macOS and other platforms...

Well in addition to macOS, I can also still reproduce the issue on Linux 
(Ubuntu 18.04, with its default GCC 7) as well, so maybe it's an issue 
with newer C++ standard headers, regardless of platform?

> > This issue can crop up e.g. when trying to build gettext for mingw (which
> > has got the gnulib isnan function bundled, even if it isn't used).
>
> I'm regularly building gettext on mingw and haven't seen this issue. Therefore
> thanks again for the reproduction example.

I ran into it while trying to build gettext with clang (with my pure-llvm 
based toolchain, at https://github.com/mstorsjo/llvm-mingw in case you're 
interested), so I presume there's something that goes different in gettext 
with clang/lld compared to gcc, that makes the few bits of C++ in gettext 
not be built and/or include gnulib's math.h with isnan. Otherwise I'm sure 
lots of others would have run into it ages ago. (Also, I only run into it 
if building a shared gettext, not with a static-only build, for some 
reason.)

// Martin