That's a fair criticism around relying on implicit type conversion mechanics, and part of the tradeoff to make. On the other hand, I imagine (and my imagination may be limited) that one downside of static_assert is to increase verbosity, something like:

auto r = f();
static_assert(std::is_same_v<decltype(r),MyReturnType>> || !is_expensive_conversion_v<MyReturnType>);
return r;

Welcome to C! Tiny suggestion to add to other comments: value is already Boolean, so there's no need to write if (value == 1), you can just write if (value). Similarly, following @chamaeleon@kbin.social's suggestion of using the ternary operator, you can write return value ? "Yes" : "No";.