In reply to stefaniecg:
Hi,
I’ve checked syntax you are using. It works fine for other tools, your questa version is not supporting the syntax!
//This works fine. Although using tick
//gives error saying "expected arguments (3) but
//provided arguments (2) only"
a_z <= {a_z[1:0],a};
Checkout same snippet on www.edaplayground.com and see if its supported by other tools as well.