The constraint can't be used for phantom types. If C is an interface
with an empty type list and P is given by
type P(type T C) struct{}
you can't instantiate P because no type can satisfy C.
C is not the type with 0 inhabitants: it's a constraint on types that
cannot be satisfied.
I don't see how it can be used in any valid program or why it would be
hard to detect. I am a little fuzzy on when type lists bring the
underlying type into play, though.
On Thu, Jun 18, 2020 at 1:17 PM Axel Wagner
<axel.wagner...@googlemail.com> wrote:
>
> Addendum: In Go, every type needs to be inhabited by at least one value -
> it's zero value. And we already have a type that can take *exactly* one
> value, namely struct{}.
>
> On Thu, Jun 18, 2020, 22:13 Axel Wagner <axel.wagner...@googlemail.com> wrote:
>>
>> These arguments would be more convincing, if Go wouldn't already reject
>> interfaces impossible to implement: https://play.golang.org/p/dYm8js26qml
>>
>> On Thu, Jun 18, 2020, 17:26 Jesper Louis Andersen
>> <jesper.louis.ander...@gmail.com> wrote:
>>>
>>> It is a type which cannot be inhabited by a term. These exist and often
>>> have uses. As Bryan wrote they also completes the type lattice, so
>>> rejecting them is often a lot of work for little gain.
>>>
>>> If you want examples, look up phantom types, where an uninhabited type is
>>> used as a tag for ensuring compile time restrictions.
>>>
>>> On Wed, Jun 17, 2020, 22:09 jimmy frasche <soapboxcic...@gmail.com> wrote:
>>>>
>>>> This isn't a bug per se, but I can file one if requested.
>>>>
>>>> https://go2goplay.golang.org/p/AWynhg6ya7h
>>>>
>>>> Since embedding interfaces with type lists uses the intersection of
>>>> the items in the type list, it's possible to create an interface that
>>>> cannot be satisfied by any type.
>>>>
>>>> Currently this does not cause an error until you attempt to
>>>> instantiate something that uses such an interface as a bound.
>>>>
>>>> I think it would be more useful to raise the error when defining the
>>>> interface that cannot be used as it's likely an error—or at least I
>>>> can see no valid use for creating an unsatisfiable constraint.
>>>>
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