Merge branch 'concepts.CxxUtils-20200503' into 'master'

CxxUtils: Add a helper to start playing with C++20 concepts.

See merge request atlas/athena!32579

Control/CxxUtils/CxxUtils/concepts.h

+// 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