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A general description is here.
Package installation, ‘R CMD INSTALL’ or ‘install.packages(*)’, now parses each of the ‘<pkg>/R/*.R’ files individually instead of first concatenating and then ‘parse()’ing the large resulting file. This allows parser or syntax errors to be diagnosed with correct file names and line numbers, thanks to Simon Dedman's report and Bill Dunlap's patch in PR#17859.
This _does_ require syntactically self contained R source files now, fixing another inadvertant bug.