diff --git a/var/spack/repos/builtin/packages/root/package.py b/var/spack/repos/builtin/packages/root/package.py
index 3900dad876..4b421b04f4 100644
--- a/var/spack/repos/builtin/packages/root/package.py
+++ b/var/spack/repos/builtin/packages/root/package.py
@@ -334,6 +334,8 @@ def determine_variants(cls, exes, version_str):
             v.append("cxxstd=14")
         elif "cxx17" in f:
             v.append("cxxstd=17")
+        elif "cxx20" in f:
+            v.append("cxxstd=20")
 
         # helper function: check if featurename is in features, and if it is,
         # append variantname to variants. featurename may be a list/tuple, in