function(x, dim=dim(x)) { str(dim) } => recursive default argument reference

Is the following a bug?

Rterm -vanilla

foo <- function(x, dim=dim(x)) { str(dim) }
foo(1:2)

Error in str(dim) :
?promise already under evaluation: recursive default argument
reference or earlier problems?

foo(x=1:2)

Error in str(dim) :
?promise already under evaluation: recursive default argument
reference or earlier problems?

But, this works:

foo <- function(x, dim2=dim(x)) { str(dim2) }
foo(1:2)

?NULL

foo <- function(x, dim=base::dim(x)) { str(dim) }
foo(1:2)

?NULL

sessionInfo()

R version 2.9.0 Patched (2009-05-28 r48680)
i386-pc-mingw32

locale:
LC_COLLATE=English_United States.1252;LC_CTYPE=English_United States.1252;LC_MON
ETARY=English_United States.1252;LC_NUMERIC=C;LC_TIME=English_United States.1252

attached base packages:
[1] stats ? ? graphics ?grDevices utils ? ? datasets ?methods ? base

/Henrik