OK, thanks. So I guess the eagerness of the template machinery is not
something you can work around. Perhaps the only way around this is to
avoid std::conjunction and explicitly encode a staged evaluation, for
instance:
template <typename T> constexpr bool is_barable_v =
std::decay_t<T>::barable::value;
template <typename T, typename = void>
struct is_fooable : std::false_type {};
template <typename T>
struct is_fooable<T, std::void_t<typename std::decay_t<T>::baz>> :
std::bool_constant<!is_barable_v<T>> {};
struct Foo {
using barable = std::false_type;
using baz = bool;
};
Best regards,
/Johan
On Wed, May 29, 2019 at 12:16 PM Jonathan Wakely <jwakely.gcc@xxxxxxxxx> wrote:
>
> On Wed, 29 May 2019 at 16:50, Johan Alfredsson wrote:
> >
> > Hi,
> >
> > I was under the impression that you could use std::conjunction to
> > avoid instantiating templates for arguments following an argument the
> > ::value of which would evaluate to something equivalent to 'false'.
>
> That's correct.
>
> > The code below seems to be an example of when this is not the case.
> > I'm I misunderstanding something or is there a bug somewhere in gcc or
> > libstdc++?
>
> I think this is a bug in your code. The is_barable_v<T> variable
> template requires the instantiation of is_barable<T>::value
> immediately, even if the conjunction instantiation doesn't need it.
