Detect UCRT-built R from within R sessions (and in configure.win)

Hi,

Are there any proper ways to know whether the session is running on
the R that is built with the UCRT toolchain or not? Checking if the
encoding is UTF-8 might do the trick, but I'm not sure if it's always
reliable.

There in not such a mechanism, yet, but can be added, at least for 
diagnostics.

You are right that checking for UTF-8 encoding would not always be 
reliable. For example, the version of Windows may be too old to allow R 
use UTF-8 as native encoding (e.g. Windows server 2016), then R will use 
the native code page as it does today in the MSVCRT builds.

Also, I'd like to know if there's any mechanism to detect the UCRT in
configure.win. I know there are Makevars.ucrt and Makefile.ucrt, but
one might want to do some feature test that is specific to the UCRT
toolchain.

We could add support for configure.ucrt, which would take precedence 
over configure.win on the UCRT builds (like Makevars.ucrt takes 
precedence over Makevars.win). Would that work for you?

Best
Tomas

Best,
Hiroaki Yutani

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