[Solved (fix in progress)] double integration of sqrt(x+y) for x and y fails

[parisse]

This is a bug, I'm fixing it in Xcas and submitting the patch to HP. Until it is in, you can do that:
f1:=int((x+y)^(1/2), x);
int(subst(f1,x=1)-subst(f1,x=0),y,0,1);