disable clang warning about pack indexing in C++20

Author: ericniebler

include/stdexec/__detail/__config.hpp

@@ -398,6 +398,15 @@ namespace stdexec {
 #  define STDEXEC_TERMINATE() std::terminate()
 #endif
 
+// Some compilers turn on pack indexing in pre-C++26 code. We want to use it if it is
+// available. Pack indexing is disabled for clang < 20 because of:
+// https://github.com/llvm/llvm-project/issues/116105
+#if defined(__cpp_pack_indexing) && !STDEXEC_NVCC() && !(STDEXEC_CLANG() && __clang_major__ < 20)
+#  define STDEXEC_HAS_PACK_INDEXING() 1
+#else // ^^^ has pack indexing ^^^ / vvv no pack indexing vvv
+#  define STDEXEC_HAS_PACK_INDEXING() 0
+#endif // no pack indexing
+
 #if STDEXEC_HAS_FEATURE(thread_sanitizer) || defined(__SANITIZE_THREAD__)
 #  define STDEXEC_TSAN(...) STDEXEC_HEAD_OR_TAIL(1, __VA_ARGS__)
 #else

include/stdexec/__detail/__meta.hpp

@@ -1025,9 +1025,10 @@ namespace stdexec {
     using __f = __mor<__minvoke<_Fn, _Args>...>;
   };
 
-// C++23 pack indexing is disabled for clang because of
-// https://github.com/llvm/llvm-project/issues/116105
-#if defined(__cpp_pack_indexing) && !STDEXEC_CLANG()
+#if STDEXEC_HAS_PACK_INDEXING()
+  STDEXEC_PRAGMA_PUSH()
+  STDEXEC_PRAGMA_IGNORE_GNU("-Wc++26-extensions")
+
   template <bool>
   struct __m_at_ {
     template <class _Np, class... _Ts>
@@ -1039,6 +1040,8 @@ namespace stdexec {
 
   template <std::size_t _Np, class... _Ts>
   using __m_at_c = __minvoke<__m_at_<_Np == ~0ul>, __msize_t<_Np>, _Ts...>;
+
+  STDEXEC_PRAGMA_POP()
 #elif STDEXEC_HAS_BUILTIN(__type_pack_element)
   template <bool>
   struct __m_at_ {