verilog: fix signedness when removing unreachable cases

CHANGELOG

@@ -5,6 +5,11 @@ List of major changes and improvements between releases
 Yosys 0.17 .. Yosys 0.17-dev
 --------------------------
 
+ * Verilog
+    - Fixed an issue where simplifying case statements by removing unreachable
+      cases could result in the wrong signedness being used for comparison with
+      the remaining cases
+
 Yosys 0.16 .. Yosys 0.17
 --------------------------
  * New commands and options

frontends/ast/simplify.cc

@@ -1531,6 +1531,7 @@ bool AstNode::simplify(bool const_fold, bool at_zero, bool in_lvalue, int stage,
 		detectSignWidth(width_hint, sign_hint);
 		while (children[0]->simplify(const_fold, at_zero, in_lvalue, stage, width_hint, sign_hint, in_param)) { }
 		if (children[0]->type == AST_CONSTANT && children[0]->bits_only_01()) {
+			children[0]->is_signed = sign_hint;
 			RTLIL::Const case_expr = children[0]->bitsAsConst(width_hint, sign_hint);
 			std::vector<AstNode*> new_children;
 			new_children.push_back(children[0]);

tests/verilog/unreachable_case_sign.ys

@@ -0,0 +1,33 @@
+logger -expect-no-warnings
+
+read_verilog -formal <<EOT
+module top(input clk);
+    reg good = 0;
+
+    always @(posedge clk) begin
+        case (4'sb1111) 15: good = 1; 4'b0000: ; endcase
+        assert (good);
+    end
+endmodule
+EOT
+
+prep -top top
+sim -n 3 -clock clk
+
+design -reset
+
+read_verilog -formal <<EOT
+module top(input clk);
+    reg good = 1;
+    reg signed [1:0] case_value = -1;
+
+    always @(posedge clk) begin
+        case (4'sb1111) 4'b0000: ; case_value: good = 0; endcase
+        assert (good);
+    end
+endmodule
+EOT
+
+prep -top top
+sim -n 3 -clock clk
+