verilog: fix $past's signedness

CHANGELOG

@@ -4,6 +4,9 @@ List of major changes and improvements between releases
 
 Yosys 0.17 .. Yosys 0.17-dev
 --------------------------
+ * Formal Verification
+    - Fixed the signedness of $past's return value to be the same as the
+      argument's instead of always unsigned.
 
  * Verilog
     - Fixed an issue where simplifying case statements by removing unreachable

frontends/ast/genrtlil.cc

@@ -1084,7 +1084,7 @@ void AstNode::detectSignWidthWorker(int &width_hint, bool &sign_hint, bool *foun
 				sub_sign_hint = true;
 				children.at(0)->detectSignWidthWorker(sub_width_hint, sub_sign_hint);
 				width_hint = max(width_hint, sub_width_hint);
-				sign_hint = false;
+				sign_hint &= sub_sign_hint;
 			}
 			break;
 		}

frontends/ast/simplify.cc

@@ -3230,6 +3230,7 @@ skip_dynamic_range_lvalue_expansion:;
 
 					reg->str = stringf("$past$%s:%d$%d$%d", filename.c_str(), location.first_line, myidx, i);
 					reg->is_reg = true;
+					reg->is_signed = sign_hint;
 
 					current_ast_mod->children.push_back(reg);
 

tests/verilog/past_signedness.ys

@@ -0,0 +1,35 @@
+logger -expect-no-warnings
+
+read_verilog -formal <<EOT
+module top(input clk);
+    reg signed [3:0] value = -1;
+    reg ready = 0;
+
+    always @(posedge clk) begin
+        if (ready)
+            assert ($past(value) == -1);
+        ready <= 1;
+    end
+endmodule
+EOT
+
+prep -top top
+sim -n 3 -clock clk
+
+design -reset
+
+read_verilog -formal <<EOT
+module top(input clk);
+    reg signed [3:0] value = -1;
+    reg ready = 0;
+
+    always @(posedge clk) begin
+        if (ready)
+            assert ($past(value + 4'b0000) == 15);
+        ready <= 1;
+    end
+endmodule
+EOT
+
+prep -top top
+sim -n 3 -clock clk