[Rd] codetools wrongly complains about lazy evaluation in S4 methods

[Mikael Jagan]

In a package, I define a method for not-yet-generic function 'qr.X' like so:

     > setOldClass("qr")
     > setMethod("qr.X", signature(qr = "qr"), function(qr, complete, ncol) NULL)

The formals of the newly generic 'qr.X' are inherited from the non-generic
function in the base namespace.  Notably, the inherited default value of
formal argument 'ncol' relies on lazy evaluation:

     > formals(qr.X)[["ncol"]]
     if (complete) nrow(R) else min(dim(R))

where 'R' must be defined in the body of any method that might evaluate 'ncol'.
To my surprise, tools:::.check_code_usage_in_package() complains about the
undefined symbol:

     qr.X: no visible binding for global variable 'R'
     qr.X,qr: no visible binding for global variable 'R'
     Undefined global functions or variables:
       R

I claim that it should _not_ complain, given that lazy evaluation is a really
a feature of the language _and_ given that it already does not complain about
the formals of functions that are not S4 methods.

Having said that, it is not obvious to me what in codetools would need to change
here.  Any ideas?

I've attached a script that creates and installs a test package and reproduces
the check output by calling tools:::.check_code_usage_in_package().  Hope it
gets through.

Mikael
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