R devel - Sep 2025 - [BUG?] S4 validity function not enforced during object creation in R 4.4.2

Dear Yingkai,

The default method for 'initialize', which is called by 'new' in
your example
and is accessible as

     > selectMethod("initialize", "ANY")

has always called 'validObject' *only if* it finds arguments matching
'...',
so the behaviour that you report is not a change from earlier behaviour.

The bug seems to be that you, the class author, defined a nonvirtual class
with an invalid prototype, *not* that 'new' ignores invalid prototypes.

I added the following regression test in Matrix version 1.6-0 in order to
catch exactly this mistake:

     for (cl in c(names(getClassDef("Matrix")@subclasses),
                 
names(getClassDef("MatrixFactorization")@subclasses)))
         if (!isVirtualClass(def <- getClassDef(cl)))
             validObject(new(def))

One could argue that setClass(name, ...) should test validObject(new(name))
before returning whenever it defines a nonvirtual class.  But I can imagine
false positives, e.g., if a class author documents that 'new' *must* be
called with additional arguments, then the validity of the prototype seems
inconsequential.  Hence the test ought to be optional and probably (at least
initially) disabled by default.

Mikael

> Date: Thu, 11 Sep 2025 09:09:25 +0800 (CST)
> From: =?gb2312?B?y+/Tor+t?=<sunyingkai at sjtu.edu.cn>
> 
> Dear R Core Team,
> I would like to report a possible bug or behavioral change in the S4 class
system in R 4.4.2 regarding the validity function.
> Description of the issue:
> In previous versions of R, when the validity function of an S4 class
returns a character string (i.e., an error message), the ?new() function would
refuse to create the object and throw an error.
> However, in R 4.4.2, I found that ?new() creates the object successfully
even when the validity function returns an error message. Only a manual call to
?validObject() triggers the error.
> Minimal reproducible example:
> 
setClass('TestVital',
>           slots = list(visit_type='character'),
>           prototype = list(visit_type=''),
>           validity = function(object){
>             if(!object at visit_type %in% c('OP','IP')){
>               return('??????')
>             }
>             TRUE
>           })
> new('TestVital')  # This should fail, but it succeeds in R 4.4.2
> validObject(new('TestVital'))  # This correctly triggers the error
> 
Session info:
> 
R version 4.4.2 (2024-10-31)
> Platform: aarch64-apple-darwin20
> Running under: macOS Sequoia 15.6.1
> 
Expected behavior:
> ?new('TestVital') should fail and throw an error if the validity
function returns a character string, as documented in the official R extensions
manual.
> Actual behavior:
> ?new('TestVital') creates the object even when the validity
function returns an error message.
> Is this an intentional change in R 4.4.x, or is it a bug?
> Thank you for your attention.
> Best regards,
> Sun Yingkai