That is a follow-up to the following commit:
> commit 6b51b12ab5
> Author: Laurent Rineau <laurent.rineau@cgal.org>
> Date: Fri Oct 4 16:42:13 2013 +0200
>
> Fix the case when FT is mpq_class
>
> If x and w are mpq_class objects, then the type of x/w is not mpq_class,
> but only a proxy type that is implicitly convertible to
> mpq_class. With the type deduction, CGAL::make_array(x/w, y/w,
> z/w) will not create an array<mpq_class> but an array of the proxy type.
>
> That creates the following compilation error, in a ternary operator:
>
> - with clang:
> include/CGAL/Cartesian/Vector_3.h:78:25: error: incompatible operand types ('array<__gmp_expr<[...], struct __gmp_binary_expr<class __gmp_expr<mpq_t, mpq_t>, class __gmp_expr<mpq_t, mpq_t>, struct __gmp_binary_divides>>, [...]>' and 'array<__gmp_expr<[...], __mpq_struct [1]>, [...]>')
> : base( w != FT_(1) ? CGAL::make_array(x/w, y/w, z/w)
> ^ ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
> - with g++:
> include/CGAL/Cartesian/Vector_3.h:78:25: error: no match for ternary 'operator?:' (operand types are 'bool', 'std::array<__gmp_expr<__mpq_struct [1], __gmp_binary_expr<__gmp_expr<__mpq_struct [1], __mpq_struct [1]>, __gmp_expr<__mpq_struct [1], __mpq_struct [1]>, __gmp_binary_divides> >, 3ul>', and 'std::array<__gmp_expr<__mpq_struct [1], __mpq_struct [1]>, 3ul>')
> : base( w != FT_(1) ? CGAL::make_array(x/w, y/w, z/w)
> ^
>
> The fix is to specify the template argument of CGAL::make_array.
The first patch in 2013 was on `Vector_3`, but `Vector_2` also suffers
from the issue.
3e4fcd8d90