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Re: CNTTP defaulted to lambda in definition produces different type every time

From: Brian Bi <bbi5291_at_[hidden]>
Date: Thu, 27 Apr 2023 11:14:26 -0400
On Thu, Apr 27, 2023 at 11:08 AM Daniel Kr├╝gler via Std-Discussion <
std-discussion_at_[hidden]> wrote:

> Am Do., 27. Apr. 2023 um 17:05 Uhr schrieb Thiago Macieira via
> Std-Discussion <std-discussion_at_[hidden]>:
> >
> > Is this intended with C++20's class-type NTTPs?
>
> No,
>
> see
>
> https://cplusplus.github.io/CWG/issues/cwg_active.html#2542


But this doesn't really answer the OP's question. Consider that we can
simply do this:

template <class T> struct Structural {};

template <auto Call = Structural<decltype([]{})>{}> struct S {};

Now the template parameter type is structural, but there is still a
question of whether the lambda type is the same every time `S<>` is
referenced.


>
> > template <auto Call = []{}> struct S {};
> > static_assert(!std::is_same_v<S<>, S<>>);
> >
> > It passes the compilation in the three major compilers that have
> implemented
> > the feature in the first place:
> > https://conformance.godbolt.org/z/edf8Pv71j
> >
> > I'm not asking to change this. I actually want it this way. But I'd like
> a
> > confirmation that this is the intention and won't suddenly break my code
> later.
>
> It is not intended and it is likely that compilers will adjust to make
> that ill-formed.
>
> - Daniel
> --
> Std-Discussion mailing list
> Std-Discussion_at_[hidden]
> https://lists.isocpp.org/mailman/listinfo.cgi/std-discussion
>


-- 
*Brian Bi*

Received on 2023-04-27 15:14:43