Hi All,

Currently, lambdas can apply attributes to annotate operator() and its type. For example, the following is valid syntax (https://godbolt.org/z/hc4Er3h9h):

  [] [[attr]] ([[attr]] int) [[attr]] {}; // ok

  [] [[attr]] () [[attr]] {};             // ok
  [] [[attr]] [[attr]] {};                // ok
  [] [[]] [[]] {};                        // ok, empty attribute

But unfortunately, attributes cannot be used in lambda capture lists:

int x = 0;
[ [[maybe_unused]] x ](int y) { return y; }; // error

I'm wondering if it would be reasonable to allow it to support attributes?
I can think of a practical use case where this is useful in the real world, which is optimizing the size of the lambda itself.
For example, the current standard std::bind_front implementation could be simplified to the following:

template<auto fn, typename... BindArgs>
constexpr auto
bind_front(BindArgs&&... bindargs) -> decltype(auto) {
  return [...boundargs(std::forward<BindArgs>(bindargs))]
         <typename Self, typename... CallArgs>
         (this Self&&, CallArgs&&... callargs) -> decltype(auto) {
           return std::invoke(fn, 
             std::forward_like<Self>(boundargs)..., std::forward<CallArgs>(callargs)...);
         };
}

constexpr int add(int x, int y) { return x + y; }
static_assert(bind_front<add>(3)(42) == 45);

However, there is one unsatisfactory aspect here, which is that the members captured by the capture list cannot be optimized for overlap, which is a common optimization in library implementations, i.e., Empty base optimization.
If we could use attributes in the capture lists, then we could simply do:

template<auto fn, typename... BindArgs>
constexpr auto
bind_front(BindArgs&&... bindargs) -> decltype(auto) {
  return [... [[no_unique_address]] boundargs(std::forward<BindArgs>(bindargs))]
         <typename Self, typename... CallArgs>
         (this Self&&, CallArgs&&... callargs) -> decltype(auto) {
           return std::invoke(fn, 
             std::forward_like<Self>(boundargs)..., std::forward<CallArgs>(callargs)...);
         };
}

So that we no longer need to make our own special storage class type to perform such optimization.
I reasonably suspect there may be many more use cases for applying attributes in lambda capture lists.

What do you think? Is this a good idea?

Hewill