Define type in generate block and use it outside - system-verilog

Question

I'm attempting to define a user-defined type in a generate block and then access it in the scope outside the generate block. Something like this:
generate
if (SOME_PARAM == 0) begin
typedef struct packed {
logic [A-1:0][B-1:0] field1;
logic [C-1:0] field2;
} struct_id_t;
end
else begin
typedef struct packed {
logic [SOME_PARAM-1:0] other_field;
logic [A-1:0][B-1:0] field1;
logic [C-1:0] field2;
} struct_id_t;
end
endgenerate
struct_id_t some_struct_instance;
The last line of code triggers an error indicating that struct_id_t is undefined. Apparently, generate blocks create a local scope, which would explain the error.
Is there another way to accomplish this?
Before discussing what I've considered/tried, I'll first say that I understand one solution to this is to place all uses of struct_id_t in the local scope created by the generate block. However, this would require quite a bit of code duplicated between the blocks that I'd really prefer to avoid.
Other things I've considered:
A compiler directive does not create a local scope like a generate block. Unfortunately, (System)Verilog does not have an ifeq compiler directive, so I don't think this will work.
I also tried to forward declare the type. Something like:
typedef struct struct_id_t;
generate
if (SOME_PARAM == 0) begin
typedef struct packed {
logic [A-1:0][B-1:0] field1;
logic [C-1:0] field2;
} struct_id_t;
end
else begin
typedef struct packed {
logic [SOME_PARAM-1:0] other_field;
logic [A-1:0][B-1:0] field1;
logic [C-1:0] field2;
} struct_id_t;
end
endgenerate
struct_id_t some_struct_instance;
The resulting error complains about struct_id_t never being defined. Section 6.18 of the 1800-2012 standard explains the error:
The actual data type definition of a forward typedef declaration shall be resolved within the same local scope or generate block.
So, the definition needs to be in the same scope.
I tried to create a label for the generate blocks that includes the parameter definition. That way I believe I could access the type hierarchically. Something along the lines of gen_name[SOME_PARAM].struct_id_t. But it seems that's not a valid name for a generate block.
What are my options?

Answer 1

There is no way to declare a type in a generate block this way. But, you can define a variable inside a block as the following and use it with a structural name:
if (SOME_PARAM == 0) begin: def
typedef struct packed {
logic [A-1:0][B-1:0] field1;
logic [C-1:0] field2;
} struct_id_t;
struct_id_t some_struct_instance;
end
else begin: def
typedef struct packed {
logic [SOME_PARAM-1:0] other_field;
logic [A-1:0][B-1:0] field1;
logic [C-1:0] field2;
} struct_id_t;
struct_id_t some_struct_instance;
end
assign def.some_struct_instance = 1;
The other approach is to pass the type to the instance of your module (mod):
module top#(SOME_PARAM = 0, A=1, B=1, C=1)();
typedef struct packed {
logic [A-1:0][B-1:0] field1;
logic [C-1:0] field2;
} struct1_id_t;
typedef struct packed {
logic [SOME_PARAM-1:0] other_field;
logic [A-1:0][B-1:0] field1;
logic [C-1:0] field2;
} struct2_d_t;
mod #(.stuct_id_t(struct1_id_t)) mod1();
mod #(.stuct_id_t(struct1_id_t)) mod2();
endmodule
module mod#(type struct_id_t = logic) ();
struct_id_t some_struct_instance;
// ...
endmodule