I suppose if there were to be an API to get the default values, it would probably be as an expression and not just a single value, right? Since defaults are materialized at the call-site, which is important for e.g. std::source_location::current(). Defaulted member variables too can incorporate other member variables, and the 'this' pointer, which would maybe warrant some thinking towards as well, even though we're talking about defaulted function parameters here.

But I think you could maybe still accomplish a std::function_defargs with only knowing whether certain parameters are defaulted. You could maybe generate an overload set that does the right thing of not providing values for the defaulted parameters for overloads of the corresponding signature, or you could probably just count the default arguments ahead of time and do an 'if constexpr' check for whether to simply forward on the passed arguments or throw an exception if not enough arguments are supplied. I don't know that you actually need to be able to get the default values, rather than just know that they exist.
On Wednesday, March 5th, 2025 at 9:27 AM, Sebastian Wittmeier via Std-Proposals <std-proposals_at_[hidden]> wrote:

> To make the default argument unique (regardless of declaration), the paper states:
>
> 10 Reflecting on Parameter Default ValuesFor completeness sake, we are mentioning default values for parameters. In our research, we have encountered
> the need to know whether a parameter has a default value, but not necessarily what the value is. Since the
> language forbids redeclaration of the same function with a default value for the same parameter, there is no
> ambiguity. Therefore, we only propose to add the has_default_argument as it was already in [P1240R2].
>
> Which does not necessarily preclude additional research and an addition for C++29.
>
>> -----Ursprüngliche Nachricht-----
>> Von: Ville Voutilainen via Std-Proposals <std-proposals_at_[hidden]>
>> Gesendet: Mi 05.03.2025 15:57
>> Betreff: Re: [std-proposals] std::function with default arguments
>> An: std-proposals_at_lists.isocpp.org;
>> CC: Ville Voutilainen <ville.voutilainen_at_[hidden]mail.com>;
>> On Wed, 5 Mar 2025 at 16:51, Thiago Macieira via Std-Proposals
>> <std-proposals_at_[hidden]> wrote:
>>>
>>> On Wednesday, 5 March 2025 06:18:17 Pacific Standard Time Frederick Virchanza
>>> Gotham wrote:
>>> > It's a bit of a mammoth, but I gave it a scroll through. Unless I've
>>> > missed something, I don't see any part where you can get a function's
>>> > default arguments.
>>>
>>> That would be the answer I'm looking for.
>>
>> https://wg21.link/p3096
>>
>> is the paper you're looking for. It's been approved by EWG and should
>> progress into spec reviews.
>> --
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