missing S3 method but higher level-function works

Question

I'm trying to pin down a problem with grobWidth applied to gtable objects (see gtable:::widthDetails.gtable); an issue arises when the size is specified as a sum of units (unit.arithmetic object),

library(grid)
u = unit(1,"npc") + unit(2,"mm")
grid:::absolute.units(u)
#Error in UseMethod("absolute.units") : 
# no applicable method for 'absolute.units' applied to an object of class #"c('unit.arithmetic', 'unit')"

Remarkably, absolute.size works, even though it calls grid:::absolute.units,

grid::absolute.size(u)
#[1] 1null+2mm

How is that possible?

Answer 1

OK, browsing the grid source code, I found out that the method is defined,

grid:::absolute.units.unit.arithmetic(u)

but it's not exported. So absolute.size() knows about it, because it's in the package namespace, but calling it from outside (e.g. gtable) fails.