fix c++20 build failure with pre-c++20 standard libraries

NEWS.md

@@ -1,5 +1,11 @@
 # RcppParallel (development version)
 
+* 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.
+
 * 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

patches/cpp20_concepts.diff

@@ -0,0 +1,34 @@
+Guard the concepts-based iterator tag dispatch in parallel_for_each on
+__TBB_CPP20_CONCEPTS_PRESENT rather than __TBB_CPP20_PRESENT. The latter only
+tests the language mode (__TBB_LANG >= 202002L), so toolchains that accept
+-std=c++20 but ship a pre-C++20 standard library -- e.g. Apple clang 14 with
+the macOS 11.3 SDK, as used on CRAN's macOS x86_64 builders -- would take the
+concepts branch and fail with 'no member named random_access_iterator in
+namespace std'.
+
+This is the upstream fix from oneTBB PR #1611 (issue #1552), first released in
+oneTBB 2022.1.0; it can be dropped once the bundled oneTBB is updated past
+2022.0.0.
+
+diff --git a/src/tbb/include/oneapi/tbb/parallel_for_each.h b/src/tbb/include/oneapi/tbb/parallel_for_each.h
+index ce68a7ff..13d945ce 100644
+--- a/src/tbb/include/oneapi/tbb/parallel_for_each.h
++++ b/src/tbb/include/oneapi/tbb/parallel_for_each.h
+@@ -409,7 +409,7 @@ public:
+ template<typename It>
+ using tag = typename std::iterator_traits<It>::iterator_category;
+
+-#if __TBB_CPP20_PRESENT
++#if __TBB_CPP20_CONCEPTS_PRESENT
+ template <typename It>
+ struct move_iterator_dispatch_helper {
+     using type = It;
+@@ -448,7 +448,7 @@ using iterator_tag_dispatch = typename
+             std::input_iterator_tag
+         >::type
+     >::type;
+-#endif // __TBB_CPP20_PRESENT
++#endif // __TBB_CPP20_CONCEPTS_PRESENT
+
+ template <typename Body, typename Iterator, typename Item>
+ using feeder_is_required = tbb::detail::void_t<decltype(tbb::detail::invoke(std::declval<const Body>(),

src/tbb/include/oneapi/tbb/parallel_for_each.h

@@ -409,7 +409,7 @@ class parallel_for_body_wrapper {
 template<typename It>
 using tag = typename std::iterator_traits<It>::iterator_category;
 
-#if __TBB_CPP20_PRESENT
+#if __TBB_CPP20_CONCEPTS_PRESENT
 template <typename It>
 struct move_iterator_dispatch_helper {
     using type = It;
@@ -448,7 +448,7 @@ using iterator_tag_dispatch = typename
             std::input_iterator_tag
         >::type
     >::type;
-#endif // __TBB_CPP20_PRESENT
+#endif // __TBB_CPP20_CONCEPTS_PRESENT
 
 template <typename Body, typename Iterator, typename Item>
 using feeder_is_required = tbb::detail::void_t<decltype(tbb::detail::invoke(std::declval<const Body>(),