Unexpected argument-matching when some are missing

When trying out some variations with `[.data.frame` I noticed some (to me) odd behaviour, which I found out has nothing to do with `[.data.frame`, but rather with the way arguments are matched, when mixing named/unnamed and missing/non-missing arguments. Consider the following example:

myfun <- function(x,y,z) {

  print(match.call())

  cat('x=',if(missing(x)) 'missing' else x, '\n')

  cat('y=',if(missing(y)) 'missing' else y, '\n')

  cat('z=',if(missing(z)) 'missing' else z, '\n')

}

myfun(x=, y=, "z's value")

gives:

# myfun(x = "z's value")

# x= z's value

# y= missing

# z= missing

This seems very counterintuitive to me, I expect the arguments x and y to be missing, and z to get ?z?s value?.

Unexpected argument-matching when some are missing

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