Verilator

Test case:

module submod(
        rst_n,
        clk,
        rst_n_ext
);

parameter STAGES = 4;
parameter NUM_OUT = 1;

input   rst_n;
input   clk;
output  [NUM_OUT-1:0] rst_n_ext;

reg[STAGES+NUM_OUT-2:0] rst_reg;

generate
genvar i;
        for (i=0; i<STAGES+NUM_OUT-1; i=i+1)
        begin
                always @(posedge clk or negedge rst_n)
                begin
                        if (~rst_n)
                                rst_reg[i] <= 1'b0;
                        else
                        begin
                                if (i==0)
                                        rst_reg[i] <= 1'b1;
                                else if (i < STAGES)
                                        rst_reg[i] <= rst_reg[i-1];
                                else
                                        rst_reg[i] <= rst_reg[STAGES-2];

                        end
                end
        end
endgenerate

        assign rst_n_ext = rst_reg[STAGES+NUM_OUT-2:STAGES-1];

endmodule

module test(
            input  rst_n,
            input  clk,
            output out
            );

   reg [4-1:0] rst1;

   submod  u1(
                .rst_n        (rst_n),
                .clk          (clk),
                .rst_n_ext    (rst1)
        );
        defparam u1.STAGES  = 5; 
        defparam u1.NUM_OUT = 4;

   reg [1-1:0] rst2;

   submod  u2(
                .rst_n        (rst_n),
                .clk          (clk),
                .rst_n_ext    (rst2)
        );
        defparam u2.STAGES = 5; 

endmodule

Error:

verilator -sp -Wno-fatal --top-module test test.v
%Error: test.v:29: Selection index out of range: 7:7 outside 4:0
%Error: test.v:29: Selection index out of range: 7:7 outside 4:0
%Error: Exiting due to 2 error(s)

Note that if any of two instantiations is commented out, there is no error.