cpp/ranges/empty

Determines whether or not has any elements.

A call to is expression-equivalent to:
 * , if that expression is valid.
 * 1) Otherwise,, if that expression is valid.
 * 2) Otherwise,, if that expression is valid and  models std.

In all other cases, a call to is ill-formed, which can result in substitution failure when  appears in the immediate context of a template instantiation.