The following module should return 0 for both outputs:
module issue016(y0, y1); output [3:0] y0; output [3:0] y1; assign y0 = -4'd1 ** -4'sd2; assign y1 = -4'd1 ** -4'sd3; endmodule
But quartus 13.1 sets y0 = 4'b0001 and y1 = 4'b1111.
Analysis: The 1st operand of the power operator is (unsigned) 4'b1111 in both cases. The 2nd operand of the power operator is self determined and its sign is not influenced by the rest of the expressions (see table 5-22 and sec. 5.5.1 of IEEE Std 1364-2005). According to table 5-6 of IEEE Std 1364-2005 this should return zero.
In my tests I have synthesized the module with:
quartus_map issue_016 --source=issue_016.v --family="Cyclone III" quartus_fit issue_016 quartus_eda issue_016 --formal_verification --tool=conformal
Crosscheck: Vivado 2013.4, XST 14.7, Isim 14.7 and Modelsim 10.1d implement this correctly.
2014-01-23 Reported via Altera mySupport (SR #11025077)
2014-01-27 Bugfix for Quartus II v14.1 prospected by Altera Support
2014-09-28 Verified that bug is still present in Quartus II v14.0
2015-05-15 Still broken in Quartus II 15.0
2017-07-01 Still broken in Quartus II 17.0