https://gcc.gnu.org/bugzilla/show_bug.cgi?id=69116
--- Comment #3 from Andrew Pinski <pinskia at gcc dot gnu.org> ---
(In reply to TC from comment #2)
> Deduction for this operator<< function template succeeds: `T` is deduced to
> be `x` from the left operand; the right operand is a non-deduced context
> because the supplied argument names a function template. Then substitution
> takes place, and we instantiate valarray<x> (whose definition is needed to
> decide if it can be constructed from std::endl), which triggers a hard error
> because you can't return or take an abstract type by value. This is not
> diagnosed in GCC 4.7.3, likely due to PR51184.
Hmm, there is a SFINAE issue here then.
We should reduce this further. A simple reduction based on the description
works still:
template<class T> struct foo {
T f();
void g(T);
};
template<class T> void operator<<(const T&, const foo<T>&) {}
template<class T> void operator<<(const T&, void (*)(void)) {}
void endl(int);
void endl(void);
struct x
{
virtual void flush() = 0;
};
void bar(x& os)
{
os << endl;
}
--- CUT ---
Also I had confused end and endl earlier which is why I had messed up.
