[Rd] S4SXP type vs S4 object bit?

[Travers Ching]

I'm trying to understand the R internals a bit better and reading over the
documentation.

I see that there is a bit related to whether an object is S4
(S4_OBJECT_MASK), and also the object type S4SXP (25).  The documentation
makes clear that these two things aren't the same.

But in practice, will the S4-bit and object type ever disagree for S4
objects?  I know that one can set the bit manually in C; are there any
practical applications for doing so?

Thank you
Travers

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