CUDA: Add issue number to Clang separable compilation error
diff --git a/Source/cmLocalGenerator.cxx b/Source/cmLocalGenerator.cxx
index f748822..3b3f110 100644
--- a/Source/cmLocalGenerator.cxx
+++ b/Source/cmLocalGenerator.cxx
@@ -1997,7 +1997,8 @@
       if (separable) {
         this->Makefile->IssueMessage(
           MessageType::FATAL_ERROR,
-          "CUDA_SEPARABLE_COMPILATION isn't supported on Clang.");
+          "CUDA_SEPARABLE_COMPILATION isn't supported on Clang. "
+          "See CMake issue #20726.");
       }
     }
   }