How can I apply the [[nodiscard]] attribute to a lambda?

Question

I want to prevent people from calling the lambda without handling the return value.

Clang 4.0 refuses everything I've tried, compiling with -std=c++1z:

auto x = [&] [[nodiscard]] () { return 1; };
// error: nodiscard attribute cannot be applied to types

auto x = [[nodiscard]] [&]() { return 1; };
// error: expected variable name or 'this' in lambda capture list

auto x [[nodiscard]] = [&]() { return 1; };
// warning: nodiscard attribute only applies to functions, methods, enums, and classes

[[nodiscard]] auto x = [&]() { return 1; };
// warning: nodiscard attribute only applies to functions, methods, enums, and classes

auto x = [&]() [[nodiscard]] { return 1; };
// error: nodiscard attribute cannot be applied to types

Is this some sort of bug in clang or a hole in the standard?

Answer 1

You can't apply nodiscard to lambdas, but you can write a wrapper:

template <typename F>
struct NoDiscard {
    F f;
    NoDiscard(F const& f) : f(f) {}
    template <typename... T>
    [[nodiscard]] constexpr auto operator()(T&&... t) const
      noexcept(noexcept(f(std::forward<T>(t)...))) {
        return f(std::forward<T>(t)...);
    }
};

int main() {
    NoDiscard([](int i) {return i;})(0);
}

Demo.