diff --git a/libcxx/utils/ci/run-buildbot b/libcxx/utils/ci/run-buildbot
--- a/libcxx/utils/ci/run-buildbot
+++ b/libcxx/utils/ci/run-buildbot
@@ -148,6 +148,13 @@
 ${CMAKE} --version
 ${NINJA} --version
 
+# When not using the CXX environment variable it's a bit harder to determine
+# the compiler selected by CMake. This depends on CMake's platform specific
+# rules. The exact compiler version is only of interest when using nightly
+# builds. (They have additional version information not displayed by CMake.)
+# These builds are always selected by the environment variable.
+if [ ! -z "${CXX}" ]; then ${CXX} --version; fi
+
 case "${BUILDER}" in
 check-format)
     clean