Why clang++37 behaves differently on 9.3 and 10.3?

[Dimitry Andric]

On 12 May 2016, at 13:33, Yuri <yuri at rawbw.com> wrote:
> 
> clang++37 compiles this simple program fine on 10.3, but fails on 9.3.
> 
> Why does it behave differently on different OS versions?
> 
> It looks like it ignores -std=c++11 on 9.3.

You cannot compile for C++11 on a 9.x installation, because clang will
use libstdc++ by default there, and the version of libstdc++ in the
9.x base system is not C++11 compatible.

To be able to use C++11, you must install libc++ first, using for
example:

export CC=clang
export CXX=clang++

cd /usr/src/lib/libcxxrt
make obj
make depend
make
make install

cd /usr/src/lib/libc++
make obj
make depend
make
make install

Alternatively, rebuild world with these settings in /etc/src.conf:

WITH_CLANG_IS_CC=y
WITH_LIBCPLUSPLUS=y

On FreeBSD 10.x, clang and libc++ are the defaults, and C++11 can be
used out of the box.


> test.cc:6:17: note: initializer of 'vmax' is not a constant expression
> test.cc:5:13: note: declared here
>  const int vmax = std::numeric_limits<int>::max();
>            ^

This is because libstdc++ defines numeric_limits<int>::max() as follows:

  template<>
    struct numeric_limits<int>
    {
...
      static int max() throw()
      { return __INT_MAX__; }

While max() appears to be pretty constant in its return value, it is not
defined constexpr, and therefore you cannot use it in a static assert.

Note that later versions of libstdc++ do use constexpr when appropriate:
https://gcc.gnu.org/viewcvs/gcc?view=revision&revision=166171

-Dimitry

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