On Wed, 13 May 2020, 22:22 Ville Voutilainen, <ville.voutilainen@gmail.com> wrote:
On Thu, 14 May 2020 at 01:35, Richard Smith via SG12
<sg12@lists.isocpp.org> wrote:
>>> The status quo in the standard is:
>>> 1) Language linkage is part of the function type.
>>> 2) Initializing a function pointer with a wrong-language-linkage function (pointer) is a type error.
>>> 3) Explicitly casting a function pointer to the wrong type and calling it is UB.

>>> (a) the status quo: the standard continues to say the same thing and vendors continue to ignore it,
>>> (b) we remove point (1) above and break the targets relying on it, or
>>> (c) we remove point (2) above.
> Wow, this is embarrassing, that's still wrong. Should be:
> """
> To me it seems like (a) and (b) are not acceptable resolutions, and EWG didn't like conditionally-supported-(c). So maybe we should be considering conditionally-supported-(b) (which would simply be standardizing existing practice).
> """

Funny. From a programmer perspective, I would prefer removing all of
points 1, 2, and 3. I may be wrong,
but removing point 1 seems to result in points 2 and 3 being removed.

That's option (b). Removing point 1 does remove point 2 but not 3 as presented: there are other ways the type can differ. (But yes, it would define away the language linkage part of point 3.)

If I program with the false hope that 1/2/3 don't exist, what target
platforms is my code currently not portable to?
What platforms is it portable to?

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.)