xgboost: GCC 8+ required (#32282)

var/spack/repos/builtin/packages/xgboost/package.py

@@ -39,6 +39,8 @@ class Xgboost(CMakePackage, CudaPackage):
     depends_on("llvm-openmp", when="%apple-clang +openmp")
     depends_on("hwloc", when="%clang")
 
+    # https://github.com/dmlc/xgboost/issues/6972
+    conflicts("%gcc@:7", when="+cuda")
     conflicts("%gcc@:4", msg="GCC version must be at least 5.0!")
     conflicts("+nccl", when="~cuda", msg="NCCL requires CUDA")
     conflicts("+cuda", when="~openmp", msg="CUDA requires OpenMP")