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Subject: Re: EWG Requests feedback/suggestions on Core Issue 1555
From: Richard Smith (richardsmith_at_[hidden])
Date: 2020-05-14 12:46:27


On Thu, 14 May 2020, 09:57 David Vandevoorde, <daveed_at_[hidden]> wrote:

>
>
> On May 14, 2020, at 4:00 AM, Richard Smith via SG12 <sg12_at_[hidden]>
> wrote:
>
> [ About language linkage being part of function types…]
>
> I hope someone who maintains an implementation for such a target will
> speak up. (I think someone did when this was last asked, though, so I don't
> think this is a theoretical or legacy concern. I'd be very happy to be
> wrong about that.)
>
>>
> Our front end can be configured in this respect:
> - Whether C and C++ language linkage function types are distinct
> - If they are not distinct, whether they can be implicitly converted
> from one to another
> (and some added configurability to decide whether those implicit
> conversions are
> enabled by default or require a command-line option)
>
> For those customers who have shared they configurations with us, most
> appear to keep the types distinct but allow implicit conversions between
> them. So the following example is accepted:
>
> $ cat -> r.cpp
> extern "C" typedef int F(void);
> extern "C++" typedef int FPP(void);
>
> constexpr int g(F) { return 1; }
> constexpr int g(FPP) { return 2; };
>
> F f;
> FPP fpp;
> F *pf = fpp;
>
> static_assert(g(f) == 1);
> static_assert(g(fpp) == 2);
> $ ./cfe --c++20 r.cpp
> $
>
>
> (The “major” implementations would complain g has a duplicate definition.)
>

So, if we want to make all known implementations conforming, we would need
to make both (1) and (2) conditionally supported. Right? I'd be less happy
with that than with making only one of the two options
conditionally-supported, but maybe it's the right choice, given that
changing only (2) would require an ABI break for major implementations to
conform, and changing only (1) would be a source-level break for Daveed's
customers described above -- that is, for the people who were trying to
follow our rules as best as they could even though their C and C++
functions had the same calling convention.

(Also, it sounds like there are not just one or two targets like this,
given Hubert's report and Daveed's multiple customers. I'm guessing the
latter implementations are things like processor-specific compilers in the
embedded space or similar, which I think generally have less representation
on the committee.)

Now we have "Recommended behavior" as a tool, we could make (1)
conditionally supported, with a recommendation that C language linkage
result in the same type as C++ language linkage, and make (2) conditionally
supported, with a recommendation to allow conversion if the calling
convention would be the same (for those that didn't follow our first
recommendation).

Daveed
>
>
>



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