CxxUtils: Add ATH_MEMBER_REQUIRES.

Add ATH_MEMBER_REQUIRES/ATH_MEMBER_REQUIRES_DEF to concepts.h.

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.
+ * Copyright (C) 2002-2021 CERN for the benefit of the ATLAS collaboration.
  */
 /**
  * @file CxxUtils/concepts.h
@@ -18,6 +18,21 @@
  *
  * The body of the ATH_REQUIRES will be hidden for compilers
  * that don't support concepts.
+ *
+ * ATH_MEMBER_REQUIRES can be used to selectively enable a member function.
+ * Example:
+ *@code
+ *  template <class T>
+ *  class C { ...
+ *
+ *    ATH_MEMBER_REQUIRES(std::is_same_v<T,int>, double) foo (double x) { ... }
+ @endcode
+ * declares a function @c foo returning @c double that is enabled
+ * only if @c T is an @c int.   If you have a definition separate
+ * from the declaration, then use ATH_MEMBER_REQUIRES_DEF in the definition
+ * instead.
+ *
+ * Will use a @c requires clause with C++20 and @c enable_if otherwise.
  */
 
 
@@ -25,15 +40,36 @@
 #define CXXUTILS_CONCEPTS_H
 
 
+#include <type_traits>
+
+
 #ifdef __cpp_concepts
 
 #include <concepts>
 #define ATH_REQUIRES(...) requires __VA_ARGS__
 
+#define ATH_MEMBER_REQUIRES(CONDITION, RETTYPE) \
+  template <bool = true> \
+  requires (CONDITION)   \
+  RETTYPE
+
+#define ATH_MEMBER_REQUIRES_DEF(CONDITION, RETTYPE) \
+  template <bool> \
+  requires (CONDITION)   \
+  RETTYPE
+
 #else
 
 #define ATH_REQUIRES(...)
 
+#define ATH_MEMBER_REQUIRES(CONDITION, RETTYPE) \
+  template <bool Enable = true> \
+  std::enable_if_t<Enable && (CONDITION), RETTYPE>
+
+#define ATH_MEMBER_REQUIRES_DEF(CONDITION, RETTYPE) \
+  template <bool Enable> \
+  std::enable_if_t<Enable && (CONDITION), RETTYPE>
+
 #endif