[ROOT/v6-18-04-40] ~ > root -l
root [0] .x test.C
root [1] .x test.C
IncrementalExecutor::executeFunction: symbol '_ZStL19__constant_string_pIcEbPKT_' unresolved while linking [cling interface function]!
You are probably missing the definition of bool std::__constant_string_p<char>(char const*)
Maybe you need to load the corresponding shared library?
Ah yes I did not noticed you ran it twice, sorry. I see the error now on Mac with the latest ROOT version. It seems a Cling issue therefore our Cling experts @Axel and @vvassilev should know the answer.