@ctfeonly

[Jonathan M Davis]

On Thursday, December 07, 2017 17:30:13 Manu via Digitalmars-d wrote:
> On 7 December 2017 at 17:20, Manu <turkeyman at gmail.com> wrote:
> > On 7 December 2017 at 13:35, Walter Bright via Digitalmars-d <
> >
> > digitalmars-d at puremagic.com> wrote:
> >> On 12/7/2017 11:41 AM, Manu wrote:
> >>> Misuse of the API; ie, a runtime call, will result in an unsuspecting
> >>> user getting a surprising link error, rather than a nice compile-time
> >>> error explaining what they did wrong...
> >>
> >> I think Nicholas is talking about functions for which code cannot be
> >> generated.
> >>
> >> In any case, in C/C++/D if you call functions in .h files (or imports)
> >> that are not linked in, you'll get a linker error.
> >
> > Right, but that's what I'm saying; using your solution of putting a
> > function in a module that's not compiled to inhibit code generation
> > won't
> > inhibit people from *attempting* to making runtime calls (and getting
> > link errors)... whereas a compile error trying to runtime-call a
> > function that shouldn't be runtime-called might be more desirable.
> > I'm not actually registering support for @ctfeonly, just that I think
> > it's a pattern that I have wanted and should be supported in some way,
> > and a compile-time error would be nicer than a link error.
>
> I tried this, and was surprised it didn't work:
>
> int ctfeOnly(int x)
> {
> static assert(__ctfe);
> return x + 1;
> }
>
> This would probably solve the problem in a satisfying way without an
> attribute?

In spite of the fact that CTFE is done at compile time, __ctfe is a runtime
thing - it's just that it's runtime from the perspective of CTFE. So, stuff
like static if or static assert doesn't work. You have to use if or a
ternary operator or some other runtime conditional, though it's a common
misunderstanding that __ctfe is used with static if, and people screw it up
all the time. I don't know why it's a runtime thing rather than a compile
time thing though. There's probably a good reason for it, but at first
glance, it seems like a rather weird choice.

- Jonathan M Davis