Reference class finalize() fails with 'attempt to apply non-function'

This bug appears intermittently in R CMD check when reference classes have
finalize methods. The problem is that garbage collection can be run after
the methods package is no longer available. It affects (periodically) the
Bioconductor AnnotationDbi package as well as packages that contain Rcpp
classes. To reproduce:

?library(methods)
?a = setRefClass("A", methods=list(finalize=function() cat("A\n")))
?b = setRefClass("B", contains="A")

repeat b = setRefClass("B", contains="A") until finalize does not run (no
garbage collection triggered during setRefClass)

?b = setRefClass("B", contains="A")
?b = setRefClass("B", contains="A")

and then

detach("package:methods")
gc()

Error in function (x) ?: attempt to apply non-function
Error in function (x) ?: attempt to apply non-function

traceback()

1: function (x)
? x$.self$finalize()(<environment>)

I believe a variant of the same type of problem generates an error

Error in function (x) ?: no function to return from, jumping to top level

also seen in AnnotationDbi and Rcpp packages

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