I am on macOS High Sierra.
I have installed R-devel as per instructions in the INSTALL file shipped with the snapshot archive.
From the command line, all seems fine:
$ which R /usr/local/bin/R $R R Under development (unstable) (2017-12-07 r73866) -- "Unsuffered Consequences" Copyright (C) 2017 The R Foundation for Statistical Computing Platform: x86_64-apple-darwin17.2.0 (64-bit) [...skipped...] > R.home()  "/usr/local/lib/R"
However, if I launched RStudio, I still get my 'old' R 3.4.2:
R version 3.4.2 (2017-09-28) -- "Short Summer" > R.home()  "/Library/Frameworks/R.framework/Resources"
This RStudio help page states that:
"RStudio scans for the R executable in the following sequence:
/usr/local/bin/R # <--- this one is valid for me!
If RStudio is not able to locate R by scanning these locations, it will fall back to using whatever version of R is located at /Library/Frameworks/R.framework/."
In addition the same help page suggests the following trick that did not work for me:
export RSTUDIO_WHICH_R=/usr/local/bin/R open -a rstudio # same 3.4.2
PS: I have also restarted my computer a few times , with no effect.
Many thanks in advance!