[Rd] c(NA, 0+1i) not the same as c(as.complex(NA), 0+1i)?

[Michael Chirico]

This is another follow-up to the thread from September "Recent changes to
as.complex(NA_real_)".

A test in data.table was broken by the changes for NA coercion to complex;
the breakage essentially comes from

c(NA, 0+1i)
# vs
c(as.complex(NA), 0+1i)

The former is the output we tested against; the latter is essentially (via
coerceVector() in C) what's generated by our data.table::shift()

However, these are now (r85472) different:

Im(c(NA, 0+1i))
# [1] NA  1
Im(c(as.complex(NA), 0+1i))
# [1] 0 1

The former matches the behavior of directly using NA_complex_:

Im(c(NA_complex_, 0+1i))
# [1] NA  1

On R4.3.2, they both match the NA_complex_ behavior:
Im(c(NA, 0+1i))
# [1] NA  1
Im(c(as.complex(NA), 0+1i))
# [1] NA 1

Is this intended behavior, does something need to be updated for c() as
well?

Certainly it's messing with my understanding of how c() behaves, e.g. in ?c

> All arguments are coerced to a common type which is the type of the
returned value

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