Re: which compiler is right (either to compile or to barf)...

[leon zadorin <leonleon77@gmail.com>]

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On Fri, Dec 2, 2022 at 10:54 AM Jonathan Wakely <jwakely.gcc@gmail.com>
wrote:

> On Thu, 1 Dec 2022 at 23:27, leon zadorin via Gcc-help
> <gcc-help@gcc.gnu.org> wrote:
> > The current thought prorcess is that std component must be allowed by the
> > standard to be instantiated with incomplete type T. And vector is allowed
> > so indeed (but I think it must be complete by the time its members, e.g.
> > via member-function invocation, are referenced)... so then it may come
> down
> > to std::optional and its implementation -- whether its ctor is
> referencing
> > any vector's members (dtor etc.) ...
> >
> > I suppose the official way would be to find whether ::std::optional may
> be
> > instantiated with incomplete type T
>
> It may not be.
>
>
>  (say even if only when using default,
> > no value present, ctor overload)... and if it not mentioned of being able
> > to do so then it would be a bug in the original code (at which point
> having
> > some toolchain being able to detect it is rather super nice :) :)
> >
> > I dont know to be honest if standard's doco for such ctor , e.g.
> >    constexpr optional() noexcept; constexpr optional(nullopt_t) noexcept;
> >          Postconditions: *this does not contain a value.
> > <https://timsong-cpp.github.io/cppwp/n4861/optional#ctor-1.sentence-1>
> >          Remarks: No contained value is initialized.
> > <https://timsong-cpp.github.io/cppwp/n4861/optional#ctor-2.sentence-1>
> >          For every object type T these constructors are constexpr
> > constructors
> >
> > would be sufficient to infer the ability to instantiate with incomplete
> > type... probably not (I'm not enough of an expert to see through this
> with
> > my cloudy head atm :)
>
> Those constructors are irrelevant. It's undefined to instantiate *any*
> std::lib template with an incomplete type unless *explicitly*
> permitted. It is explicitly permitted for vector and shared_ptr. It is
> not permitted for optional.
>

Thanks Jonathan for clarifying, as always - very precise and helpful. I'll
close the issue on the clang side also.