Can a noexcept function still call a function that throws in C++17?

Question

In P0012R1, \"Make exception specifications be part of the type system\",
I see that noexcept is now becoming a part of the function type.

I

Answer 1

noexcept(true) functions can call noexcept(false) functions. There will be a runtime error if an exception is thrown. The canonical example of why this is allowed is:

double hypotenuse(double opposite, double adjacent) noexcept(true)
{
    return std::sqrt(opposite*opposite + adjacent*adjacent);
}

std::sqrt will throw domain_error if its argument is negative, but clearly that will never happen here.

(In an ideal world, it would be forbidden by default with an exception_cast to allow it where required. The result could either be UB if an exception is thrown, or std::terminate).