I think the evil of needing the explicit opt-in significantly outweighs the perceived freedom it gives library authors. Note that as I've already mentioned, there are numerous properties of functions that the user can observe but which are likely implementation details, such as:

recursive leakage through headers
expression validity tests / SFINAE stability
ability to form function pointers and function references
ability to pass template arguments explicitly, especially cv-qualified types
any property only obtainable through reflection (such as parameter names)
lack or existence of default function arguments
lack or existence of default template arguments
distinct declaration vs using-declaration of an existing global entity
various type choices, like using iterators as pointers
existence of a noexcept specification or lack thereof

Sometimes these are stable, but very often it's not intended for users to rely on them. This has been the status quo of the last 30 years, and people talk about adding function parameter names to this list as if it was obviously unacceptable, but it really isn't.

On the other hand, the ability to rely on function parameter names is not really a problem in practice in languages with support for named arguments, like Kotlin. It's only conjectured to be a major problem in C++. However, an explicit opt-in would require every library author to do significant work to permit passing named function arguments, and this work isn't exactly clean and easy. Except for fresh C++29-only codebases, this would devolve into code looking like:

~~f(int x, int y);~~

int MY_LIB_NAMED_SINCE_CXX29(x),

int MY_LIB_NAMED_SINCE_CXX29(x)

);

I don't want to see every code base looking like this for the next 10-20 years, or authors understandably not bothering with named parameters because this looks disgusting. There could still be a per-function opt-in rather than a per-parameter opt-in, but I'm not sure if that's worth the trouble.

On Sun, 4 Jan 2026 at 15:42, Sebastian Wittmeier via Std-Proposals <std-proposals@lists.isocpp.org> wrote:

Parameter names are optional in function declarations. (And can be named differently from their counterpart in definitions.)
So far they were either
- kept the same between declaration and definition -> leak of internal (implementation) data
- used as reminder for the programmer or for popups and autocompletes in IDEs; possibly also for compile error messages

So the programmer already has a lot of flexibility. Nevertheless you are right, that the parameter names of included functions never leaked into the caller code up till now.

-----Ursprüngliche Nachricht-----
Von: Barry Revzin via Std-Proposals <std-proposals@lists.isocpp.org>
Gesendet: So 04.01.2026 15:13
Betreff: Re: [std-proposals] Labelled parameters
An: std-proposals@lists.isocpp.org;
CC: Barry Revzin <barry.revzin@gmail.com>;

Even when the parameter names are normal-ish, that's probably not something that library authors want users to rely on. For instance, in range-v3, the predicates and projections are pretty consistently named pred and proj, but is that the correct user-facing name? Eric and Casey have never had to think about this before. max(r, .proj=key) is definitely nicer to read than max(r, {}, key), but not as nice as max(r, .key=key) (or ".by" or something else). Lack of explicit opt-in cements a decision that up until now was never even a known decision.

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