genrtlil: add width detection for AST_PREFIX nodes

frontends/ast/genrtlil.cc

@@ -993,6 +993,14 @@ void AstNode::detectSignWidthWorker(int &width_hint, bool &sign_hint, bool *foun
 		break;
 	}
 
+	case AST_PREFIX:
+		// Prefix nodes always resolve to identifiers in generate loops, so we
+		// can simply perform the resolution to determine the sign and width.
+		simplify(true, false, false, 1, -1, false, false);
+		log_assert(type == AST_IDENTIFIER);
+		detectSignWidthWorker(width_hint, sign_hint, found_real);
+		break;
+
 	case AST_FCALL:
 		if (str == "\\$anyconst" || str == "\\$anyseq" || str == "\\$allconst" || str == "\\$allseq") {
 			if (GetSize(children) == 1) {

tests/simple/loop_prefix_case.v

@@ -0,0 +1,18 @@
+module top(
+	input wire x,
+	output reg y
+);
+	localparam I = 1;
+	genvar i;
+	generate
+		for (i = 0; i < 1; i = i + 1) begin : blk
+			wire [i:i] z = x;
+		end
+	endgenerate
+	always @* begin
+		case (blk[I - 1].z)
+			1: y = 0;
+			0: y = 1;
+		endcase
+	end
+endmodule