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A general description is here.
This update was mainly bug fixes but contained a barely documented major change. The set of BLAS routines had been unchanged since 1988, so throughout R's history. This update introduced new BLAS routines ‘dgemmtr’ and ‘zgemmtr’ which are now used by LAPACK routines. This means that BLAS implementations are no longer interchangeable.
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 will.
A C23 compiler is known to be selected with ‘gcc’ 13-15, 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.
There are new functions ‘asRboolean’ and ‘asBool’, variants of ‘asLogical’ more suited to converting logical arguments to ‘Rboolean’ or to ‘bool’. They require a length-one input and throw an error if that evaluates to ‘NA’.