modelsim compiling error:external function 'xxxxx' may not be used in a constant expr

Let's say you have this

package pkg;
int gacc;
function int fout(input int in);
  int acc;
  if (in !=0) begin
      acc += in;
      fout = acc+ gacc;
      gacc = acc;
  end
endfunction
endpackage
// in some code you have
$display(fout(1)); // prints 1
$display(fout(0)); // prints 1 - returns last value assigned to fout
$display(fout(1)); // prints 3

Those results are because gacc, add, in, and fout are all static variables initialized once at time 0.

You can make a function automatic, which makes all arguments, return values, and local variables implicitly automatic (initialized upon each entry to the function)

package pkg;
int gacc;
function automatic int fout(input int in);
  int acc; // implicitly automatic
  if (in !=0) begin
      acc += in;
      fout = acc+ gacc;
      gacc = acc;
  end
endfunction
endpackage
// in some code you have
$display(fout(1)); // prints 1
$display(fout(0)); // prints 0 // no value assigned to fout
$display(fout(1)); // prints 2

When you declare a package or module as automatic, then all named blocks, tasks, and functions declared in that package become implicitly automatic

package automatic pkg;
int gacc; // always static - not inside any block
function int fout(input int in); // implicitly automatic
  int acc; // implicitly automatic
  if (in !=0) begin
      acc += in;
      fout = acc+ gacc;
      gacc = acc;
  end
endfunction
endpackage
// in some code you have
$display(fout(1)); // prints 1
$display(fout(0)); // prints 0
$display(fout(1)); // prints 2