diff --git a/test/lit.cfg b/test/lit.cfg
index aa454397..5e64e54d 100644
--- a/test/lit.cfg
+++ b/test/lit.cfg
@@ -181,10 +181,11 @@ if cxx_under_test is None:
     cxx_under_test = getattr(config, 'cxx_under_test', None)
 
     # If no specific cxx_under_test was given, attempt to infer it as clang++.
-    clangxx = lit.util.which('clang++', config.environment['PATH'])
-    if clangxx is not None:
-        cxx_under_test = clangxx
-        lit_config.note("inferred cxx_under_test as: %r" % (cxx_under_test,))
+    if cxx_under_test is None:
+        clangxx = lit.util.which('clang++', config.environment['PATH'])
+        if clangxx is not None:
+            cxx_under_test = clangxx
+    lit_config.note("inferred cxx_under_test as: %r" % (cxx_under_test,))
 if cxx_under_test is None:
     lit_config.fatal('must specify user parameter cxx_under_test '
                      '(e.g., --param=cxx_under_test=clang++)')