Casting (convert) structure to array of bits: bit-stream type error

Question

In systemverilog LRM, there is a sample code to explain casting. When I try this code, there is an error.

typedef struct {
   bit isfloat;
   union { int i; shortreal f; } n; // anonymous type
} tagged_st; // named structure

typedef bit [$bits(tagged_st) - 1 : 0] tagbits;
tagged_st a [7:0]; // unpacked array of structures
tagbits t = tagbits'(a[3]); / convert structure to array of bits
a[4] = tagged_st'(t); // convert array of bits back to structure

• First, for $bits() function, compiler says the argument of the system function call was not of bit-stream type.
• Second, when assign a[3] with a type casting of tagbits, it says The source of the target of the bit-stream casting is not of bit-stream type
  My understanding is that, structure and unpacked array are also bit-stream type.
  Hope to know what is a point I missed.(cadence 18.09-006)

Answer 1

An unpacked union is not a bitstream type. From the LRM,

By default, a union is unpacked, meaning there is no required representation for how members of the union are stored

This means you cannot know how many bits are represented.