check for whether a type instance can be streamed

Question

I've puzzled over this meta-function for a long time. It seems to work, but I suspect it to contain UB, when it checks for size of a possibly undefined reference type? Is there a problem with this meta-function?

template <class S, class C, typename = void>
struct is_streamable : ::std::false_type { };

template <class S, class C>
struct is_streamable<S,
  C,
  decltype(void(sizeof(decltype(::std::declval<S&>() <<
    ::std::declval<C const&>()))))
> : ::std::true_type
{
};

EDIT: The motivation for my question (and worries) was this question. Why did he not use a similar trick (checking size of a reference type)?

Answer 1

when it checks for size of a possibly undefined reference type?

That never induces UB. It would just result in a deduction failure (according to [temp.deduct]/8), causing the primary template to be instantiated. [expr.sizeof]:

When applied to a reference or a reference type, the result is the size of the referenced type.

The sizeof operator shall not be applied to an expression that has […] incomplete type […].

But a yet incomplete ostream is regarded "streamable" for a string if the global operator<< is overloaded.
To fix that, define the partial specialization as

template <class S, class C>
struct is_streamable<S, C,
    decltype(void( std::declval<S&>() << std::declval<C const&>() ))
    > : ::std::true_type
{};

Demo.