Only use [[fallthrough]] in C++11 and beyond

[jdemeyer]

In this code:

  #ifdef __cplusplus
    #if __has_cpp_attribute(fallthrough)
      #define CYTHON_FALLTHROUGH [[fallthrough]]
    #elif __has_cpp_attribute(clang::fallthrough)
      #define CYTHON_FALLTHROUGH [[clang::fallthrough]]
    #endif
  #endif

Cython checks for the presence of the fallthrough attribute but that doesn't mean that the C++11 attribute syntax [[fallthrough]] is understood.

GCC 7.2.0 for example gives warnings when compiling with -std=gnu++98:

build/cythonized/sage/libs/lcalc/lcalc_Lfunction.cpp:515:48: warning: c++11 attributes only available with -std=c++11 or -std=gnu++11
       #define CYTHON_FALLTHROUGH [[fallthrough]]
                                                ^
build/cythonized/sage/libs/lcalc/lcalc_Lfunction.cpp:3428:9: note: in expansion of macro 'CYTHON_FALLTHROUGH'
         CYTHON_FALLTHROUGH;
         ^~~~~~~~~~~~~~~~~~

[dalcinl]

@jdemeyer Shouldn't you keep the outer ifdef __cplusplus?

#ifdef __cplusplus
  #if __cplusplus >= 201103L
    ....
  #endif
#endif

[dalcinl]

Also, it would be good to have this in an upcoming patch release of 0.27.2, thus the merge branch should be retargeted?.

[scoder]

the merge branch should be retargeted?

Not sure how Robert thinks about this, but I generally prefer pull requests for master. Backporting or cherry picking to maintenance branches should generally be decided explicitly by those who maintain the branch.

[dalcinl]

I patched the petsc4py tarball, a new build is triggered: conda-forge/petsc4py-feedstock#4 Let see if this patch fixes the issue with clang on Travis-CI.

[dalcinl]

@scoder Sorry, I'm OK with that. It is just that I'm used to a different workflow.

[robertwb]

My workflow is generally to patch the release then merge back into master over cherry-picking (for stuff that I know is safe) but given that we historically haven't maintained release branches vs. just issued new releases (deciding on a major vs. minor bump based on its contents) and have only one "active" branch except when actively preparing a release the difference is small.