According to the standard, the behavior is undefined if [s, s+count) is not a valid range (even though the constructor may not access any of the elements of this range).
At the time, libcxx gives runtime assertion in case of invokation of std::string_view ctor will nullptr passed.
This patch improves the diagnostic of the described case by making the compilation fail.
The same applies to std::string ctors. I will update the patch if the idea would proof useful.