diff --git a/CMakeLists.txt b/CMakeLists.txt
index 9e7e7f1fe..bceae25a0 100644
--- a/CMakeLists.txt
+++ b/CMakeLists.txt
@@ -21,6 +21,14 @@ INCLUDE(GNUInstallDirs)
 SET(CMAKE_CXX_STANDARD 20)
 SET(CMAKE_CXX_STANDARD_REQUIRED True)
 
+# GCC older than 11 accepts the configure but fails deep in the build (CUB, the C++20 headers), e.g.
+# the system GCC 8 of RHEL/Rocky 8; stop here instead and say which compiler to use.
+IF (CMAKE_CXX_COMPILER_ID STREQUAL "GNU" AND CMAKE_CXX_COMPILER_VERSION VERSION_LESS 11)
+    MESSAGE(FATAL_ERROR "GCC ${CMAKE_CXX_COMPILER_VERSION} is too old: GCC 11 or newer is required. "
+            "On RHEL/Rocky 8 install gcc-toolset-13 and run 'source /opt/rh/gcc-toolset-13/enable' "
+            "before cmake, in a fresh build directory.")
+ENDIF()
+
 # The Metal Bragg integration engine (image_analysis/bragg_integration) is Objective-C++. Enabled here,
 # above every target that links it, as enable_language asks.
 IF (APPLE)
diff --git a/docs/SOFTWARE.md b/docs/SOFTWARE.md
index 183b41846..3f5edfc91 100644
--- a/docs/SOFTWARE.md
+++ b/docs/SOFTWARE.md
@@ -22,7 +22,7 @@ CUDA requirements of each released package.
 
 ## Software dependencies
 Required:
-* C++20 compiler and C++20 standard library; recommended GCC 11+ or clang 14+ (Intel OneAPI and AMD AOCC also work)
+* C++20 compiler and C++20 standard library: GCC 11 or newer (checked at configure time; on RHEL/Rocky 8 use gcc-toolset-13), or clang 14+ (Intel OneAPI and AMD AOCC also work)
 * CMake version 3.26 or newer + a build tool (GNU make or Ninja)
 
 HDF5, libtiff, zlib and Eigen used to be required system packages; they are now