I checked and found PetscComplex was correctly defined to std::complex, and clang-6.0 was also fine to use GNU C++ library.  The problem was the gcc-4.8.5 C++ library clang picked up was a bit old, which conformed to C++11. However clang-6.0's default uses C++14, which changed the rules of constexpr.

Workarounds include:

• define PETSC_SKIP_CXX_COMPLEX_FIX  in the offending *.cxx file.
• add CXXOPTFLAGS=-std=c++11 
• update clang-6.o or gcc-4.8.5 (of 2015) on that machine. 

--Junchao Zhang

On Fri, May 14, 2021 at 10:42 AM Satish Balay <balay@mcs.anl.gov> wrote:

You can login to: isdp001.cels.anl.gov - using your MCS account credentials [and then setup ~/.ssh/authorized_keys]

Satish

On Fri, 14 May 2021, Junchao Zhang wrote:

> Satish, how to access this machine? I want to know why complex is screwed
> up.
>
> --Junchao Zhang
>
>
> On Thu, May 13, 2021 at 7:08 PM Matthew Knepley <knepley@gmail.com> wrote:
>
> > Nope. I will use your fix.
> >
> >   Thanks,
> >
> >      Matt
> >
> > On Thu, May 13, 2021 at 7:55 PM Matthew Knepley <knepley@gmail.com> wrote:
> >
> >> I am going to try just including petscsys.h and see if it works.
> >>
> >>   Thanks,
> >>
> >>      Matt
> >>
> >> On Thu, May 13, 2021 at 6:23 PM Satish Balay <balay@mcs.anl.gov> wrote:
> >>
> >>> This gets the build going...
> >>>
> >>> diff --git a/src/sys/dll/cxx/demangle.cxx b/src/sys/dll/cxx/demangle.cxx
> >>> index 31810ea15f..793a97d285 100644
> >>> --- a/src/sys/dll/cxx/demangle.cxx
> >>> +++ b/src/sys/dll/cxx/demangle.cxx
> >>> @@ -1,3 +1,4 @@
> >>> +#define PETSC_SKIP_COMPLEX
> >>>  #include <petsc/private/petscimpl.h>
> >>>
> >>>  #ifdef PETSC_HAVE_CXXABI_H
> >>>
> >>> Satish
> >>>
> >>> On Thu, 13 May 2021, Satish Balay wrote:
> >>>
> >>> > >         CXX arch-ci-linux-clang-avx/obj/sys/dll/cxx/demangle.o
> >>> >
> >>> > It is built with a c++ compiler - so __cplusplus should be defined.
> >>> [PETSC_HAVE_CXXABI is not defined]
> >>> >
> >>> > Do you need to build this sourcefile file in a clanguage=C build?
> >>> >
> >>> > I'm not sure if a c++/complex build  is checked with this compiler.
> >>> >
> >>> > [eventhough its clang build - I see the compiler is using system
> >>> incldues aka from gcc-4.8.5 - so perhaps some things don't work?]
> >>> >
> >>> > One option is to add the following to this sourcefile:
> >>> >
> >>> > #define PETSC_SKIP_COMPLEX
> >>> >
> >>> > Satish
> >>> >
> >>> >
> >>> > On Thu, 13 May 2021, Matthew Knepley wrote:
> >>> >
> >>> > > In this CI run (linux-clang-avg):
> >>> > >
> >>> > >   https://gitlab.com/petsc/petsc/-/jobs/1260342204
> >>> > >
> >>> > > The compile fails building a C++ file, demangle.cxx. It fails at the
> >>> first
> >>> > > line,
> >>> > > including <petsc/private/petscimpl.h>, down in petscsystypes.h. It
> >>> bombs
> >>> > > during the definition of complex because it looks like the compiler
> >>> is not
> >>> > > defining __cplusplus, and thus takes the wrong branch. Is this what
> >>> is
> >>> > > happening?
> >>> > > I cannot access this machine.
> >>> > >
> >>> > >   Thanks,
> >>> > >
> >>> > >       Matt
> >>> > >
> >>> > >
> >>> >
> >>> >
> >>>
> >>>
> >>
> >> --
> >> What most experimenters take for granted before they begin their
> >> experiments is infinitely more interesting than any results to which their
> >> experiments lead.
> >> -- Norbert Wiener
> >>
> >> https://www.cse.buffalo.edu/~knepley/
> >> <http://www.cse.buffalo.edu/~knepley/>
> >>
> >
> >
> > --
> > What most experimenters take for granted before they begin their
> > experiments is infinitely more interesting than any results to which their
> > experiments lead.
> > -- Norbert Wiener
> >
> > https://www.cse.buffalo.edu/~knepley/
> > <http://www.cse.buffalo.edu/~knepley/>
> >
>