Using typedef with wire in SystemVerilog

Question

When using typedef to declare a user-defined type, both these forms are accepted by EDA Playground:

typedef reg[4:0] reg5;
typedef logic[4:0] logic5;

However, if doing something similar based on wire type, then this format fails:

typedef wire[4:0] wire5;

I get "Syntax Error".

How can that be explained?

Answer 1

It is illegal to declare a typedef with a wire type, according to the IEEE Std 1800-2017. Refer to section 6.18 User-defined types:

type_declaration ::=                // from A.2.1.3 
typedef data_type type_identifier { variable_dimension } ;

A wire is not a data_type, whereas reg and logic are. A wire is a net_type.

Answer 2

A typedef defines a datatype that can be applied to a net or variable object kinds. Things are confusing to maintain backward compatibility with Verilog because of implicit definitions.

When you see

logic [4:0] v;

This is implicitly a variable declaration a with an explicit 5-bit 4-state packed array datatype.

var logic [4:0] v;

When you see

wire [4:0] w;

This is explicitly a net declaration a with a 5-bit implicit 4-state packed array datatype.

wire logic [4:0] w;

You can use a typedef for both:

typedef logic [4:0] uint5;

var  uint5 v; // var can be implicit
wire uint5 w;

Note that module input and inout ports have wire as the implicit object kind. Also reg and logic are synonyms for 4-state types with logic being the preferred keyword. A reg is not always associated with a register.