On Wed, Apr 2, 2025 at 10:45 AM Vladimir Grigoriev via Std-Discussion <
std-discussion_at_[hidden]> wrote:

> It just means that the provided quote in the C++ Standard and other
> similar quotes are incorrect. There must be included somehow the term
> «templated entities» to make the quotes clear what they are saying about.
> In my opinion it is a serious defect of the Standard because in other place
> of the Standard there is written:
>
> *4 The optional requires-clause in an init-declarator or member-declarator
> shall be present only if the declarator declares a templated function.*
>

I don't think this is a Standard defect. A non-templated lambda with
requires-clause is syntactically valid. The call operator of its closure
type has that same requires-clause. But that call operator is semantically
invalid because of that very rule you quoted ([dcl.decl.general] p4). So
the program is ill-formed. The existing Standard wording is sufficient.

There are plenty of other semantic requirements which must be checked for a
closure type or its members, where the lambda expression itself wouldn't
violate the wording of the same rule.

-- Andrew Schepler


>
> The provided by me quote should be looked like for example:
>
> «...The trailing requires-clause of a templated entity that is either the
> function call operator or operator template is the requires-clause of the
> lambda-declarator, if any.»
>
> With best regards
> (Vlad from Moscow)
>
>
> You can meet me at http://cpp.forum24.ru/ or www.stackoverflow.com or
> http://ru.stackoverflow.com
>
>
>
> Среда, 2 апреля 2025, 1:02 +03:00 от Christof Meerwald <cmeerw_at_[hidden]
> >:
>
> On Wed, Apr 02, 2025 at 12:33:51AM +0300, Vladimir Grigoriev via
> Std-Discussion wrote:
> >
> > In the paragraph #3 of the section «7.5.5.1 Closure types» of the C++20
> Standard there is written
> >
> > «...The trailing requires-clause of the function call operator or
> operator template is the requires-clause of the lambda-declarator, if any.»
> >
> > So can a non-generic lambda have a trailing requires clause? And if the
> text of the quote is correct why is not there an example of such a lambda?
>
> Sure, it just needs to be templated, e.g.
>
> template<bool B>
> void f()
> {
> [] () requires B { } ();
> }
>
>
> There isn't an example for every feature in the standard as the
> standard is not a tutorial. Anything surprising or non-obvious should
> be highlighted by having an example in the standard.
>
>
> Christof
>
> --
> https://cmeerw.org sip:cmeerw at cmeerw.org
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>
>
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