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Mandates explicit -std=c++XY for c++ projects

Dear all,

Since the GCC 6 release [1], the default mode for C++ is now
-std=gnu++14 instead of -std=gnu++98. What this means is that upon
(re)compilation a library written for c++98 will be recompiled using a
different c++ standard (c++14 in this case), unless of course the
upstream package explicitly set the -std= flags with the appropriate
c++ version.

The ISO committee generally describe the change in between different
standards [2] and in some case, one can find examples of subtle change
in behaviors [3] and [4].

With this mind I'd like to make mandatory the -std=c++XY flags when
compiling either a c++ library or a stand-alone c++ program:

1. Either upstream define the explicit -std=c++XY flags by mean of its
build system,
2. Or the package maintainers needs to explicit change the CXXFLAGS to
pass the appropriate version of the c++ standard. In which case this
should be documented in the README.Debian file.
3. As a fallback, dh should initialize the CXXFLAGS with -std=gnu++98

If there is a consensus on the following change, I'll go ahead and
also file a bug for lintian to scan the compilation logs in search for
missing -std=c++ expression when g++ command line are issued.

Thanks for your comments,

[1] https://gcc.gnu.org/gcc-6/changes.html
[2] https://isocpp.org/files/papers/p0636r0.html
[3] https://stackoverflow.com/questions/23980929/what-changes-introduced-in-c14-can-potentially-break-a-program-written-in-c1
[4] https://stackoverflow.com/questions/46650351/what-changes-between-c98-and-c11-show-difference-in-behavior

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