Re-enable all examples

CMakeLists.txt

@@ -8,8 +8,11 @@ project(
   DESCRIPTION "A Beman library exemplar"
   LANGUAGES CXX)
 
+list(APPEND CMAKE_MODULE_PATH "${CMAKE_CURRENT_LIST_DIR}/cmake")
 include(CTest)
 include(FetchContent)
+include(CompilerFeatureTest)
+beman_check_range_support(COMPILER_SUPPORTS_RANGES)
 
 if(BUILD_TESTING)
   enable_testing()

cmake/CompilerFeatureTest.cmake

@@ -0,0 +1,23 @@
+# # SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
+
+# Functions that determine compiler capabilities
+
+include(CheckCXXSourceCompiles)
+
+# Determines if the selected C++ compiler has ranges support. 
+# Sets 'result_var' to whether support is detected.
+function(beman_check_range_support result_var)
+  # Check if the C++ standard is at least C++20 or later.
+  if(CMAKE_CXX_STANDARD LESS 20)
+    set(${result_var} FALSE PARENT_SCOPE)
+    return()
+  endif()
+
+  check_cxx_source_compiles("
+// example specific check due to https://github.com/beman-project/exemplar/issues/41
+#include <ranges> // C++20 ranges; note that __cpp_lib_ranges is not defined for all compilers
+int main(){ return 0; }
+" _HAVE_RANGE_SUPPORT )
+
+  set(${result_var} ${_HAVE_RANGE_SUPPORT} PARENT_SCOPE)
+endfunction()

examples/CMakeLists.txt

@@ -1,8 +1,11 @@
 # SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
 
-set(ALL_EXAMPLES
-  #identity_as_default_projection
-identity_direct_usage)
+set(ALL_EXAMPLES identity_direct_usage)
+if (COMPILER_SUPPORTS_RANGES)
+  list(APPEND ALL_EXAMPLES identity_as_default_projection)
+else()
+  message(WARNING "Compiler does not support ranges. Skip: identity_as_default_projection")
+endif()
 
 foreach(example ${ALL_EXAMPLES})
   add_executable(beman.exemplar.examples.${example})