From b8a9c1b1bd04cf103f68df96cb4b3232a2bda329 Mon Sep 17 00:00:00 2001
From: scott snyder <sss@karma>
Date: Sat, 2 May 2020 18:30:09 +0200
Subject: [PATCH] CxxUtils: Add a helper to start playing with C++20 concepts.

Add header concepts.h with an ATH_REQUIRES macro, to hide a requires clause
from compilers that don't support concepts.
---
 Control/CxxUtils/CxxUtils/concepts.h | 40 ++++++++++++++++++++++++++++
 1 file changed, 40 insertions(+)
 create mode 100644 Control/CxxUtils/CxxUtils/concepts.h

diff --git a/Control/CxxUtils/CxxUtils/concepts.h b/Control/CxxUtils/CxxUtils/concepts.h
new file mode 100644
index 00000000000..2aa50223e22
--- /dev/null
+++ b/Control/CxxUtils/CxxUtils/concepts.h
@@ -0,0 +1,40 @@
+// This file's extension implies that it's C, but it's really -*- C++ -*-.
+/*
+ * Copyright (C) 2002-2020 CERN for the benefit of the ATLAS collaboration.
+ */
+/**
+ * @file CxxUtils/concepts.h
+ * @author scott snyder <snyder@bnl.gov>
+ * @date Apr, 2020
+ * @brief Compatibility helpers for using some pieces of C++20
+ *        concepts with older compilers.
+ *
+ * Use ATH_REQUIRES for a requires clause:
+ *@code
+ *  template <class T>
+ *    ATH_REQUIRES( std::assignable_from<T, float> )
+ *  void foo (T x);
+ @endcode
+ *
+ * The body of the ATH_REQUIRES will be hidden for compilers
+ * that don't support concepts.
+ */
+
+
+#ifndef CXXUTILS_CONCEPTS_H
+#define CXXUTILS_CONCEPTS_H
+
+
+#ifdef __cpp_concepts
+
+#include <concepts>
+#define ATH_REQUIRES(...) requires __VA_ARGS__
+
+#else
+
+#define ATH_REQUIRES(...)
+
+#endif
+
+
+#endif // not CXXUTILS_CONCEPTS_H
-- 
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