XSim implements 0 ** -1 incorrectly

The following module should return 8'bx for both outputs (see table 5-6 of IEEE Std 1364-2005):

module issue_019(y0, y1);
  output [7:0] y0;
  output [7:0] y1;
  assign y0  = 8'sd0 ** -8'sd1;
  assign y1  = 8'd 0 ** -8'sd1;
  initial #10 $display("%b %b", y0, y1);
endmodule

But XSim 2013.4 returns 8'b0 instead.

In my tests I have run this module with:

xvlog issue_019.v
xelab -R work.issue_019

Crosscheck: Modelsim 10.1d implements this correctly. Isim 14.7 has the same bug.

History:
2014-01-24 Reported bug in Xilinx Support Forum
2014-04-16 Still broken in XSim 2014.1
2015-05-15 Fixed in XSim 2015.1