lead the release notes with the macos install failure

RcppParallel 6.1.1 currently fails to install on r-release-macos-x86_64,
which is the most consequential thing 6.2.0 fixes. The NEWS entry framed
it as affecting code that includes the header; src/tbb.cpp includes it
via RcppParallel/TBB.h, so it broke our own compilation too.

Author: kevinushey

NEWS.md

@@ -1,5 +1,15 @@
 # RcppParallel 6.2.0
 
+* Fixed a failure to install with toolchains that accept `-std=c++20` but
+  provide a pre-C++20 standard library, with errors of the form "no member
+  named 'random_access_iterator' in namespace 'std'" from
+  `tbb/parallel_for_each.h`. The bundled oneTBB selected its concepts-based
+  iterator dispatch on a macro testing only the language standard, rather than
+  the `__TBB_CPP20_CONCEPTS_PRESENT` used elsewhere in that same header. This
+  affected RcppParallel's own compilation, as well as any package including
+  the header; CRAN's macOS x86_64 machines, which pair Apple clang 14 with the
+  macOS 11.3 SDK, are one such toolchain. (#268)
+
 * On Windows, RcppParallel now builds the bundled oneTBB as a shared library
   and links against it, shipping `tbb.dll` and `tbbmalloc.dll` alongside the
   package -- the same arrangement already used on every other platform.
@@ -53,12 +63,6 @@
   the package -- for a pre-built binary, the Rtools tree of the machine that
   built it. Both now describe the installation actually in use. (#270, #273)
 
-* Fixed an issue where compiling code including `tbb/parallel_for_each.h`
-  could fail with toolchains that accept `-std=c++20` but provide a
-  pre-C++20 standard library, with errors of the form "no member named
-  'random_access_iterator' in namespace 'std'". This affected CRAN's macOS
-  x86_64 machines, which pair Apple clang 14 with the macOS 11.3 SDK. (#268)
-
 * Fixed an issue where building the bundled oneTBB could fail when `CXX`
   (or `CC`) was configured with a leading compiler launcher such as `ccache`
   (e.g. `CXX = "ccache g++"`). The launcher is now forwarded to cmake via

cran-comments.md

@@ -1,8 +1,26 @@
 ## RcppParallel 6.2.0
 
-This release follows closely on 6.1.1. We apologize for the quick turnaround;
-it repairs a regression that leaves Windows users of RcppParallel unable to
-build some downstream packages.
+This release follows closely on 6.1.1, and we apologize for the quick
+turnaround. It fixes the installation failure currently reported for 6.1.1 on
+r-release-macos-x86_64, along with a regression that leaves Windows users
+unable to build some downstream packages from source.
+
+### Installation failure on r-release-macos-x86_64
+
+RcppParallel 6.1.1 fails to install on that machine, with:
+
+    no member named 'random_access_iterator' in namespace 'std'
+
+The bundled oneTBB guarded a concepts-based iterator dispatch in
+`parallel_for_each.h` on a macro that only tests the language standard. With a
+toolchain that accepts `-std=c++20` but provides a pre-C++20 standard library
+-- Apple clang 14 with the macOS 11.3 SDK, as on that check machine -- it
+therefore reached for `std::random_access_iterator`, which is not available.
+oneTBB already defines `__TBB_CPP20_CONCEPTS_PRESENT` as 0 for clang, and
+guards the rest of that header on it; this release uses it for the dispatch
+block as well, so the pre-C++20 fallback is selected as intended.
+
+### Downstream builds on Windows
 
 Since 6.0.0, RcppParallel has linked the static TBB provided by Rtools on
 Windows. That makes the TBB version, and its ABI, a property of the user's