Thanks for the detailed explanation. 👍

I now see the key is that the using-declaration hides the class-scope
member functions. It has, in effect, changed the order of scope lookup. I
missed this point.

On Sat, 29 May 2021 at 23:22, Ville Voutilainen <ville.voutilainen_at_[hidden]>
wrote:

> And for what it's worth, the rule is at
> http://eel.is/c++draft/basic.lookup.argdep#1
>
> Let's run through it:
>
> >unqualified lookup ([basic.lookup.unqual]) for the name in the
> unqualified-id does not find any
> >declaration of a class member, or
>
> Lookup doesn't find the member any more, because it finds the
> using-declaration and stops. So now ADL
> is enabled.
>
> >function declaration inhabiting a block scope, or
>
> ..and wasn't disabled because we wrote a using-declaration, not a
> block-scope function declaration..
>
> >declaration not of a function or function template
>
> ..and since we did declare a function, ADL remains enabled. If I do
> using swap = int; in CompletelyUnrelated,
> ADL is disabled because the using-declaration declares a non-function.
> But as long as it declares a function,
> even if non-viable, ADL is enabled, and is not disabled by finding the
> member.
>
> Disclaimer: I'm no name lookup expert, but this seems like a plausible
> explanation of how the rule works. :P
>

-- 
Yongwei Wu
URL: http://wyw.dcweb.cn/