- A glvalue of type “cv1 T1” can be cast to type “rvalue reference to cv2 T2” if “cv2 T2” is reference-compatible with “cv1 T1”. The result refers to the object or the specified base class subobject thereof. If T2 is an inaccessible or ambiguous base class of T1, a program that necessitates such a cast is ill-formed.
- Otherwise, an expression e can be explicitly converted to a type T using a static_cast of the form static_- cast<T>(e) if the declaration T t(e); is well-formed, for some invented temporary variable t. [...]
Currently when checking p3 Clang will diagnose static_cast<T&&>(e) as invalid if the argument is not reference compatible with T. However I believe the correct behavior is to also check p4 in those cases. For example:
double y = 42; static_cast<int&&>(y); // this should be OK. 'int&& t(y)' is well formed
Note that we still don't check p4 for non-reference-compatible types which are reference-related since T&& t(e); should never be well formed in those cases.