https://gcc.gnu.org/bugzilla/show_bug.cgi?id=69116
--- Comment #7 from Jonathan Wakely <redi at gcc dot gnu.org> ---
The problem overload is this one:
template<typename _Tp>
inline _Expr<_BinClos<__shift_left, _Constant, _ValArray, _Tp, _Tp>, typename
__fun<__shift_left, _Tp>::result_type>
operator <<(const _Tp& __t, const valarray<_Tp>& __v)
{
typedef _BinClos<__shift_left, _Constant, _ValArray, _Tp, _Tp> _Closure;
typedef typename __fun<__shift_left, _Tp>::result_type _Rt;
return _Expr<_Closure, _Rt>(_Closure(__t, __v));
}
That deduces _Tp as x and so tries to instantiate valarray<x> to see if __v can
be initialized from std::endl, which causes the "invalid abstract return type"
outside the immediate context.
It can be fixed using enable_if (or __gnu_cxx::__enable_if since this is C++98
code) to reject invalid types for valarray, maybe using __is_abstract. I just
need to figure out the best place to put it, and how many of the other
overloaded operators are affected.
It might make sense to make the first argument a non-deduced context, so we
only use this operator when the RHS is really a valarray, rather than matching
for anything on the LHS and then trying to convert the RHS to a valarray. I
don't know if that would break real code using valarray.
