feat: Minimal example for using C++20 concepts

Core/CMakeLists.txt

@@ -5,9 +5,15 @@ configure_file(
 # source files will be added later
 add_library(
   ActsCore SHARED "")
+
 target_compile_features(
   ActsCore
   PUBLIC ${ACTS_CXX_STANDARD_FEATURE})
+
+if(ACTS_CONCEPTS_SUPPORTED)
+  target_compile_definitions(ActsCore PUBLIC ACTS_CONCEPTS_SUPPORTED)
+endif()
+
 target_include_directories(
   ActsCore
   PUBLIC

Core/include/Acts/Geometry/SurfaceVisitorConcept.hpp

@@ -0,0 +1,25 @@
+// This file is part of the Acts project.
+//
+// Copyright (C) 2023 CERN for the benefit of the Acts project
+//
+// This Source Code Form is subject to the terms of the Mozilla Public
+// License, v. 2.0. If a copy of the MPL was not distributed with this
+// file, You can obtain one at http://mozilla.org/MPL/2.0/.
+
+#pragma once
+
+#include "Acts/Surfaces/Surface.hpp"
+
+#if defined(ACTS_CONCEPTS_SUPPORTED)
+#include <concepts>
+
+namespace Acts {
+
+template <typename T>
+concept SurfaceVisitor = requires(T v) {
+  {v(std::declval<const Surface*>())};
+};
+
+}  // namespace Acts
+
+#endif

Core/include/Acts/Geometry/TrackingGeometry.hpp

@@ -11,7 +11,9 @@
 #include "Acts/Definitions/Algebra.hpp"
 #include "Acts/Geometry/GeometryContext.hpp"
 #include "Acts/Geometry/GeometryIdentifier.hpp"
+#include "Acts/Geometry/SurfaceVisitorConcept.hpp"
 #include "Acts/Geometry/TrackingVolume.hpp"
+#include "Acts/Utilities/Concepts.hpp"
 #include "Acts/Utilities/Logger.hpp"
 
 #include <memory>
@@ -97,7 +99,7 @@ class TrackingGeometry {
   ///
   /// @param visitor The callable. Will be called for each sensitive surface
   /// that is found
-  template <typename visitor_t>
+  template <ACTS_CONCEPT(SurfaceVisitor) visitor_t>
   void visitSurfaces(visitor_t&& visitor) const {
     highestTrackingVolume()->template visitSurfaces<visitor_t>(
         std::forward<visitor_t>(visitor));

Core/include/Acts/Geometry/TrackingVolume.hpp

@@ -13,12 +13,14 @@
 #include "Acts/Geometry/GeometryContext.hpp"
 #include "Acts/Geometry/GeometryIdentifier.hpp"
 #include "Acts/Geometry/Layer.hpp"
+#include "Acts/Geometry/SurfaceVisitorConcept.hpp"
 #include "Acts/Geometry/Volume.hpp"
 #include "Acts/Material/IVolumeMaterial.hpp"
 #include "Acts/Surfaces/BoundaryCheck.hpp"
 #include "Acts/Surfaces/Surface.hpp"
 #include "Acts/Utilities/BinnedArray.hpp"
 #include "Acts/Utilities/BoundingBox.hpp"
+#include "Acts/Utilities/Concepts.hpp"
 #include "Acts/Utilities/Frustum.hpp"
 #include "Acts/Utilities/Logger.hpp"
 #include "Acts/Utilities/Ray.hpp"
@@ -245,7 +247,7 @@ class TrackingVolume : public Volume {
   ///
   /// If a context is needed for the vist, the vistitor has to provide this
   /// e.g. as a private member
-  template <typename visitor_t>
+  template <ACTS_CONCEPT(SurfaceVisitor) visitor_t>
   void visitSurfaces(visitor_t&& visitor) const {
     if (!m_confinedVolumes) {
       // no sub volumes => loop over the confined layers

Core/include/Acts/Utilities/Concepts.hpp

@@ -0,0 +1,21 @@
+// This file is part of the Acts project.
+//
+// Copyright (C) 2022 CERN for the benefit of the Acts project
+//
+// This Source Code Form is subject to the terms of the Mozilla Public
+// License, v. 2.0. If a copy of the MPL was not distributed with this
+// file, You can obtain one at http://mozilla.org/MPL/2.0/.
+
+#pragma once
+
+#if defined(ACTS_CONCEPTS_SUPPORTED)
+
+#define ACTS_REQUIRES(x) requires(x)
+#define ACTS_CONCEPT(x) x
+
+#else
+
+#define ACTS_REQUIRES(x)
+#define ACTS_CONCEPT(x) typename
+
+#endif

cmake/ActsCompilerOptions.cmake

@@ -10,9 +10,11 @@ set(ACTS_CXX_FLAGS_MINSIZEREL "")
 set(ACTS_CXX_FLAGS_RELEASE "")
 set(ACTS_CXX_FLAGS_RELWITHDEBINFO "")
 
+set(ACTS_CXX_STANDARD 17)
 set(ACTS_CXX_STANDARD_FEATURE cxx_std_17)
 if(DEFINED CMAKE_CXX_STANDARD)
   if(${CMAKE_CXX_STANDARD} GREATER_EQUAL 17)
+    set(ACTS_CXX_STANDARD ${CMAKE_CXX_STANDARD})
     set(ACTS_CXX_STANDARD_FEATURE "cxx_std_${CMAKE_CXX_STANDARD}")
   else()
     message(ERROR "CMAKE_CXX_STANDARD=${CMAKE_CXX_STANDARD}, but ACTS requires C++ >=17")
@@ -53,3 +55,25 @@ set(CMAKE_MACOSX_RPATH 1)
 set(CMAKE_INSTALL_RPATH_USE_LINK_PATH TRUE)
 # set relative library path for ACTS libraries
 set(CMAKE_INSTALL_RPATH "\$ORIGIN/../${CMAKE_INSTALL_LIBDIR}")
+
+if(${ACTS_CXX_STANDARD} GREATER_EQUAL 20)
+  file(WRITE
+    "${CMAKE_BINARY_DIR}${CMAKE_FILES_DIRECTORY}/CMakeTmp/concepts.cpp"
+    "#include <concepts>\n"
+    "template<class T, class U>\n"
+    "concept Derived = std::is_base_of<U, T>::value;\n"
+    "struct A {}; struct B : public A {};"
+    "int main() { static_assert(Derived<B, A>, \"works\");  }\n" )
+
+  message(CHECK_START "Are C++20 concepts supported")
+  try_compile(ACTS_CONCEPTS_SUPPORTED "${CMAKE_BINARY_DIR}"
+      "${CMAKE_BINARY_DIR}${CMAKE_FILES_DIRECTORY}/CMakeTmp/concepts.cpp"
+      CXX_STANDARD 20
+      OUTPUT_VARIABLE __OUTPUT)
+
+  if(ACTS_CONCEPTS_SUPPORTED)
+    message(CHECK_PASS "yes")
+  else()
+    message(CHECK_FAIL "no")
+  endif()
+endif()