Feature request: expand `S7_inherits` to accept additional S7 classes for `class` parameter

[dgkf]

S7_inherits currently requires that the class parameter be a <S7_class>, which excludes many of the useful class_*es exported by S7 such as:

S7_base_class

> S7_inherits(1L, class_integer)
#> Error in S7::S7_inherits(1L, class_integer) :
#>  `class` must be an <S7_class> or NULL

S7_S3_class

> S7_inherits(structure(list(), class = "test"), new_S3_class("test"))
#> Error in S7_inherits(structure(list(), class = "test"), new_S3_class("test")) :
#>  `class` must be an <S7_class> or NULL

S7_union

This one's definitely more debatable. If not S7_inherits, having an alternative that allows individual value type checking using the same internals that S7 would use for dispatch, such as S7_is(1L, union_integer_or_character)

> union_integer_or_character <- new_union(class_integer, class_character)
> S7_inherits(1L, union_integer_or_character)
#> Error in S7_inherits(1L, union_integer_or_character) :
#>  `class` must be an <S7_class> or NULL

S7_any

These next two might seem silly, but if we have higher-kinded classes that accept classes as properties, then allowing them to use classes without having to handle specific cases is very useful.

> S7_inherits("test", class_any)
#> Error in S7_inherits("test", class_any) :
#>  `class` must be an <S7_class> or NULL

S7_missing

> x <- bquote()
> missing(x)
#> [1] TRUE
> S7_inherits(x, class_missing)
#> Error in S7_inherits(x, class_missing) :
#>  `class` must be an <S7_class> or NULL

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^{I've been on a deep dive using S7 for a package recently and am carving out time to file feedback today. Sorry for the influx of new issues - loving the package overall 🙌}

[lawremi]

I think we want people to be using inherits(), which has similarly limited support for the various S7 class types. See for example #521.

[dgkf]

Ah, I see - either way would be fine by me, although I'm not sure if I'd expect S7_any and S7_union to be handled by inherits if the goal is to be consistent with base::inherits for other class systems.

Do you have plans for a function that would accommodate type checking against these union types?

Since nameOfClass was only added in 4.3.0, it might still be nice to support this via S7_inherits for the specialized case where the class is known to be S7.

[lawremi]

S4 implemented unions as superclasses so is() supports unions, as does inherits() for S4 objects. is() also supports "ANY". So there is some precedence. Given that S7 unions are not superclasses, we might need a new verb. So far I've been using the internal class_inherits() for that, but maybe we need to co-opt is() for union membership and ANY?

[sebffischer]

I just also ran into the "problem" of S7_inherits() not "working" for unions, so making is() work for it would be nice!