[2026-03 LWG Motion 16] P3948R1 constant_wrapper is the only tool needed

[eisenwave]

Fixes #8850
Fixes NB FR-019-210 and FR-021-218 (C++26 CD).

Also fixes https://github.com/cplusplus/nbballot/issues/779
Also fixes https://github.com/cplusplus/nbballot/issues/788

[eisenwave]

@jwakely @mattkretz we have goofed HARD. The paper deletes constant_arg_t but not the deduction guides that function_ref has for it.

https://eel.is/c++draft/func.wrap.ref.deduct

(will deal with overfull hboxes once home btw)

[jwakely]

Suggested change

	then \tcode{f != nullptr} is \tcode{true}, and
	then \tcode{f.value != nullptr} is \tcode{true}, and

[jwakely]

@jwakely @mattkretz we have goofed HARD. The paper deletes constant_arg_t but not the deduction guides that function_ref has for it.

Can we just delete that (which is fine as an editorial change, because the draft is inconsistent if it refers to a type which no longer exists), or does it need to be changed to use constant_wrapper?

[eisenwave]

I believe the purpose of the deduction guides is to make things possible like

void f();
// CTAD
std::function_ref ref = std::nontype<f>;

Or now equivalently with std::cw<f>. If this no longer works as a result of deleting the deduction guides, that seems like an inadvertent design change.

I suppose the only editorial way to get the draft into a working state is to delete the deduction guides editorially and to reintroduce them by LWG issue. Any LWG resolutions won't be merged into the DIS anyway, right? We need to ship these motions.

[zhihaoy]

Herb reminded us that accepting motion can also introduce editorial change, so I suggest just finishing the renaming here.

[Mick235711]

Personally, I support treating this as an editorial wording defect in the proposal, as the intention is obviously to replace all usage of constant_arg mechanically. In other similar renaming proposals, if the author left over a few instances in the wording, they are also editorially applied without LWG issues as long as the intention is clear when polling the motions.