[Rd] `match.call` and dots substitution

[brodie gaslam]

I'm noticing some interesting behavior in `match.call` in some corner-ish cases that arise when you try to use `match.call` to match a "grandparent" function and there are dots involved:


    fun0 <- function(a, ...) fun1(...)
       fun1 <- function(b, ...) fun2()
    fun2 <- function()
      match.call(
              fun1, sys.call(sys.parent()), expand.dots=FALSE, 

        envir=parent.frame(2)
      )
    fun0(1, 2, 3)
    
    ## fun1(b = 2, ... = pairlist(3))
    
    fun0((1), (2), (3))   # the parens make the substituted args language

    ## fun1(b = ..1, ... = pairlist(..2))


When the values in dots that need to be substituted are language, they get substituted by `..n` where `n` is the position in dots.  When they are scalars they are substituted with the scalar.  It appears this is done in 'R-3.4.0:src/main/unique.c at 1319' in `subDots`:

    while (TYPEOF(t) == PROMSXP)
     	    t = PREXPR(t);
   	if( isSymbol(t) || isLanguage(t) )
      SETCAR(b, installDDVAL(i));
    else
      SETCAR(b, t);


I'm not sure why it is necessary to use `installDDVAL`, which creates the `..n` symbols, instead of duplicating the language object and attaching that to the matched call.  Certainly from a user perspective I would prefer to see:

    ## fun1(b = (2), ... = pairlist((3)))

instead of:

       ## fun1(b = ..1, ... = pairlist(..2))


Do others agree?  Is there a structural reason why this cannot be done?  Note this behavior has been this way since at least R3.0 and probably earlier.


Best regards,

B.

PS: sessionInfo():

R version 3.4.0 (2017-04-21)
Platform: x86_64-apple-darwin15.6.0 (64-bit)
Running under: macOS Sierra 10.12.3

Matrix products: default
BLAS: /Library/Frameworks/R.framework/Versions/3.4/Resources/lib/libRblas.0.dylib
LAPACK: /Library/Frameworks/R.framework/Versions/3.4/Resources/lib/libRlapack.dylib

locale:
[1] en_US.UTF-8/en_US.UTF-8/en_US.UTF-8/C/en_US.UTF-8/en_US.UTF-8

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

loaded via a namespace (and not attached):
[1] compiler_3.4.0