This blog is updated daily.
A general description is here.
A C23 compiler (if available) is now selected by default for compilation of R and packages. R builds can opt out _via_ the ‘configure’ flag ‘--without-C23’, unless the specified or default (usually ‘gcc’) compiler defaults to C23: ‘gcc’ 15 does.
A C23 compiler is known to be selected with ‘gcc’ 13-15 (as used on Windows), LLVM ‘clang’ 18-20 (and 15 should), Apple ‘clang’ 15-17 and Intel 2024.2-2025.0 (and 2022.2 should).
Current binary distributions on macOS use Apple ‘clang’ 14 and so do not use C23.