VLSI DV Interview Puzzles · All levels

Invariance of Parameterized Type Assignment

Why does assigning `agent#(req_ext)` to `agent#(req_base)` fail even though `req_ext extends req_base`?

Puzzle

Difficulty: Hard · Puzzle 4 of 6 · Topic: Parameterized Class Puzzles

Why does assigning `agent#(req_ext)` to `agent#(req_base)` fail even though `req_ext extends req_base`?

Code

systemverilog

class req_base; endclass
class req_ext extends req_base; endclass

class agent #(type REQ_T = req_base);
  REQ_T last_req;
endclass

initial begin
  agent#(req_ext)  a_ext = new();
  agent#(req_base) a_base;
  a_base = a_ext;
end

Hint

Parameterized classes are invariant in type parameters unless language/library defines covariance.

Step-by-step solution

diagram

1) `agent#(req_ext)` and `agent#(req_base)` are different unrelated specialization types.
2) Subtyping of `req_ext` does not imply subtyping of enclosing parameterized class.
3) Assignment is type-incompatible and should fail compile-time type checking.

Answer

Answer: Assignment fails because specialization types are invariant; `agent#(req_ext)` is not a subtype of `agent#(req_base)`.

Why candidates get it wrong

Candidates transfer container covariance expectations from other languages into SV/UVM.

Interviewer follow-up

What wrapper/interface pattern would you use if you need heterogeneous specialization storage?

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