From d0f983f9427634a8e3c36f2a4d827c1284bd7370 Mon Sep 17 00:00:00 2001
From: jzmaddock <john@johnmaddock.co.uk>
Date: Wed, 29 Jul 2020 18:57:56 +0100
Subject: [PATCH] Header <execution> may not be available if it depends on
 Intel's TBB.

---
 include/boost/config/stdlib/libstdcpp3.hpp | 8 ++++++++
 1 file changed, 8 insertions(+)

diff --git a/include/boost/config/stdlib/libstdcpp3.hpp b/include/boost/config/stdlib/libstdcpp3.hpp
index e73bb410..b1466d9e 100644
--- a/include/boost/config/stdlib/libstdcpp3.hpp
+++ b/include/boost/config/stdlib/libstdcpp3.hpp
@@ -319,6 +319,14 @@ extern "C" char *gets (char *__s);
 #elif __cplusplus <= 201103
 #  define BOOST_NO_CXX14_HDR_SHARED_MUTEX
 #endif
+//
+// <execution> has a dependency to Intel's thread building blocks:
+// unless these are installed seperately, including <execution> leads
+// to inscrutable errors inside libstdc++'s own headers.
+//
+#if !__has_include(<execution>) || (!__has_include(<tbb/blocked_range.h>) && !defined(_GLIBCXX_USE_TBB_PAR_BACKEND))
+#define BOOST_NO_CXX17_HDR_EXECUTION
+#endif
 #elif __cplusplus < 201402 || (BOOST_LIBSTDCXX_VERSION < 40900) || !defined(BOOST_LIBSTDCXX11)
 #  define BOOST_NO_CXX14_HDR_SHARED_MUTEX
 #endif