simplify $bits on simple part selects

src/Convert/DimensionQuery.hs

@@ -136,4 +136,11 @@ convertBits (Right e) =
                 Nothing -> Number "32"
                 Just 0  -> Number "32"
                 Just idx -> Number $ take idx n
+        Range expr mode range ->
+            BinOp Mul size $ convertBits $ Right $ Bit expr (Number "0")
+            where
+                size = case mode of
+                    NonIndexed   -> rangeSize range
+                    IndexedPlus  -> snd range
+                    IndexedMinus -> snd range
         _ -> DimsFn FnBits $ Left $ TypeOf e

test/basic/part_select_bits.sv

@@ -0,0 +1,13 @@
+module top;
+    parameter A = 3;
+    parameter B = 4;
+    logic [A*B-1:0] arr;
+    initial begin
+        arr = 0;
+        for (integer i = 0; i < A; ++i) begin
+            // yes this is silly but it captures an interesting edge case
+            arr[i * B +: B] = $bits(arr[i * B +: B])'(i);
+        end
+        $display("%b", arr);
+    end
+endmodule

test/basic/part_select_bits.v

@@ -0,0 +1,14 @@
+module top;
+    parameter A = 3;
+    parameter B = 4;
+    reg [A*B-1:0] arr;
+    initial begin : foo
+        integer i;
+        arr = 0;
+        for (i = 0; i < A; ++i) begin
+            arr[i * B +: B] = i;
+        end
+        $display("%b", arr);
+    end
+endmodule
+