fix premature elaboration of single element patterns

src/Convert/ExprUtils.hs

@@ -36,6 +36,7 @@ simplifyStep (UniOp LogNot (BinOp Ne a b)) = BinOp Eq a b
 simplifyStep (UniOp UniSub (UniOp UniSub e)) = e
 simplifyStep (UniOp UniSub (BinOp Sub e1 e2)) = BinOp Sub e2 e1
 
+simplifyStep (e @ (Concat [Pattern{}])) = e
 simplifyStep (Concat [e]) = e
 simplifyStep (Concat es) = Concat $ filter (/= Concat []) es
 simplifyStep (Repeat (Dec 0) _) = Concat []

test/basic/struct_unit_array.sv

+module top;
+    typedef struct packed {
+        logic a, b;
+    } T;
+    typedef struct packed {
+        logic x;
+        T y;
+    } S;
+
+    localparam WIDTH = 1;
+    S [WIDTH] s = '{
+        '{x: 1, y: '{a: 1, b: 0}}
+    };
+
+    initial #1 $display("%b", s);
+endmodule

test/basic/struct_unit_array.v

+module top;
+    wire [2:0] s = 3'b110;
+    initial #1 $display("%b", s);
+endmodule