How do I implement a Parametrizable Mux in SystemVerilog?

Question

I am getting the following error in System Verilog with VCS synthesizer:

The following access has an invalid number of indices. bus[i]

Answer 1

Accessing an instances of an arrayed interface can only be accessed via a simulation constant (parameter, genvar, or hard-coded number). Data types and design elements both use dotted names to access there respected member or hierarchical-name, but the rules for accessing index arrays are different. Best description I can quickly find is in IEEE Std 1800-2012 § 23.6 Hierarchical names and § 23.7 Member selects and hierarchical names.

Here are two possible solutions:

Tri-state solution: floating if select is 0, x on the bit with multiple conflicting drivers.

module myMux
    #(int unsigned WIDTH=3)
    (
        my_interface              bus[WIDTH-1:0],
        input  logic [WIDTH-1:0]  select,
        output wire  [31:0]       out_data
    );
  for (genvar i=0; i < WIDTH; i++) begin : loop
    assign out_data = select[i] ? bus[i].in_data : 'z;
  end
endmodule

Priority selector: using a local 2D array to map the interface instances. This map can be accessed in an always block. If you are on FPGA, this is the better solution as it doesn't need tri-state.

module myMux
    #(int unsigned WIDTH=3)
    (
        my_interface              bus[WIDTH-1:0],
        input  logic [WIDTH-1:0]  select,
        output logic [31:0]       out_data
    );
  logic [31:0] map [WIDTH];
  for (genvar i=0; i < WIDTH; i++) begin : loop
    assign map[i] = bus[i].in_data;
  end
  always_comb begin
    out_data = 'x;
    for(int unsigned i=0; i<WIDTH; i++) begin
      if (select[i]) out_data = map[i];
    end
  end
endmodule