Merge branch 'cleanup/guard-concept-use' into 'master'

[cleanup] Guard use of concept in concepts/*.hh headers

See merge request core/dune-common!1305

dune/common/concepts/container.hh

@@ -5,6 +5,18 @@
 #ifndef DUNE_COMMON_CONCEPTS_CONTAINER_HH
 #define DUNE_COMMON_CONCEPTS_CONTAINER_HH
 
+// check whether c++20 concept can be used
+#if __has_include(<version>) && __has_include(<concepts>)
+  #include <version>
+  #if  __cpp_concepts >= 201907L && __cpp_lib_concepts >= 202002L
+    #ifndef DUNE_ENABLE_CONCEPTS
+    #define DUNE_ENABLE_CONCEPTS 1
+    #endif
+  #endif
+#endif
+
+#if DUNE_ENABLE_CONCEPTS
+
 #include <concepts>
 #include <iterator>
 
@@ -106,4 +118,6 @@ requires(T a, const T ca, typename T::size_type i)
 
 } // end namespace Dune::Concept
 
+#endif // DUNE_ENABLE_CONCEPTS
+
 #endif // DUNE_COMMON_CONCEPTS_CONTAINER_HH

dune/common/concepts/hashable.hh

@@ -5,6 +5,18 @@
 #ifndef DUNE_COMMON_CONCEPTS_HASHABLE_HH
 #define DUNE_COMMON_CONCEPTS_HASHABLE_HH
 
+// check whether c++20 concept can be used
+#if __has_include(<version>) && __has_include(<concepts>)
+  #include <version>
+  #if  __cpp_concepts >= 201907L && __cpp_lib_concepts >= 202002L
+    #ifndef DUNE_ENABLE_CONCEPTS
+    #define DUNE_ENABLE_CONCEPTS 1
+    #endif
+  #endif
+#endif
+
+#if DUNE_ENABLE_CONCEPTS
+
 #include <concepts>
 #include <cstddef>
 #include <functional>
@@ -40,4 +52,6 @@ concept Hashable = requires(T a)
 
 } // end namespace Dune::Concept
 
+#endif // DUNE_ENABLE_CONCEPTS
+
 #endif // DUNE_COMMON_CONCEPTS_HASHABLE_HH