Bug in S7 printer when implementing `dim` and `constructor`

[sebffischer]

library(S7)
A = new_class("A",
  constructor = function() {
    new_object(A)
  }
)

a = A()
method(dim, A) <- function(x) {
  1L
}
a
#> <A>
#> Error in attributes(object) <- list(names = names(object), dim = dim(object)): invalid first argument, must be vector (list or atomic)

^{Created on 2025-08-19 with reprex v2.1.1}

[sebffischer]

So the problem is that

S7/R/class.R

Line 305 in db59642

attributes(object) <- list(names = names(object), dim = dim(object))

fails when object is a function.

[lawremi]

The constructor needs to pass an instance of the parent class to new_object(), not the class A. Strangely, this results in A() returning a function, rather than an ordinary object. Fixing this to

A = new_class("A",
  constructor = function() {
    new_object(S7_object())
  }
)

a = A()
method(dim, A) <- function(x) {
  1L
}
a

avoids the issue. That said, we should still fix the case where A derives from class_function or other types that are incompatible with the dim attribute.

More importantly, we should make it harder for people to make this mistake with custom constructors. See #519.