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Subject: Re: SG7 Digest, Vol 11, Issue 2
From: Andrew Sutton (asutton.list_at_[hidden])
Date: 2020-11-02 21:48:07


Those look like bugs to me.

On Mon, Nov 2, 2020, 8:11 PM David Rector <davrec_at_[hidden]> wrote:

>
> > Date: Mon, 2 Nov 2020 11:00:36 -0500
> > From: Andrew Sutton <asutton.list_at_[hidden]>
> > To: Hana Dus?kov? <hanicka_at_[hidden]>
> > Cc: sg7_at_[hidden]
> > Subject: Re: [SG7] Incomplete types
> > Message-ID:
> > <
> CAFVAEf1EJ6xFxJKD0OVd_OWRaoLGQA+30eL6LAnQ0u5XGvcuNQ_at_[hidden]>
> > Content-Type: text/plain; charset="utf-8"
> >
> >> This seems counterintuitive to me. I consider `sizeof` and `decltype` a
> >> poor version of a reflection and you can do this:
> >>
> >> struct X {
> >>
> >> int a;
> >> static constexpr unsigned size = sizeof(X::a);
> >> using type = decltype(X::a);
> >>
> >> };
> >>
> >> https://compiler-explorer.com/z/9f7WKn
> >>
> >> Even when the type is not yet complete. I personally think we should
> >> support such cases.
> >>
> >
> > Default member initializers are parsed in a complete-class-context",
> > essentially meaning the parsing is delayed until the closing } of the
> > class. This is why lookup works in those cases. The same is true of
> default
> > arguments, inline member function bodies, and noexcept-specifiers.
> >
> > My mental model of injection via conseval blocks is very simple. First,
> > consteval blocks have the guarantee that they are executed where they
> > appear or where they're instantiated if they appear in a template. After
> > the metaprogram runs, the compiler effectively "pastes" the injected code
> > over top of the metaprogram as if you had manually typed in. Of course,
> > whatever rules and restrictions apply to hand-written code also apply to
> > metaprograms.
>
> This is a good point to consider on its own. While I agree with the
> mental model that injection should result in "pasting" the contents into
> the enclosing context, given the different late-parsing behavior you
> describe, a user would get the following behavior, which probably won't
> quite seem like pasting:
>
> https://cppx.godbolt.org/z/rd6KT6
>
> Perhaps a better way to put it is that model is that an executing
> metaprogram is just like *parsing* the contained declarations into the
> enclosed context, after resolving any dependencies. Parsing into a class
> does two things: 1. adds the essential content of the declarations
> immediately, and 2. defers parsing of the non-essentials until the closing
> brace of the outermost enclosing class. Evaluation of a metaprogram should
> probably result in both distinct effects as well.
>
>
>



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