Uninit error on multi-dimensional Vector

[quark17]

The following code:

import Vector::*;

(* synthesize *)
module mkTest();
  rule r;
    Vector#(2,Vector#(4,Bit#(8))) v;
    for (Integer i=0; i<2; i=i+1)
      for (Integer j=0; j<4; j=j+1)
        v[i][j] = 0;
    $display("%h", v);
  endrule
endmodule

results in this error, when synthesized (with -verilog, say):

Error: "Test.bsv", line 6, column 35: (G0028)
  `v' uses uninitialized value (the position shown is the object's
  declaration). If this error is unexpected, please consult KPNS #32.
  During elaboration of the body of rule `r' at "Test.bsv", line 5, column 8.
  During elaboration of `mkTest' at "Test.bsv", line 4, column 8.

I thought there was already an issue filed for this, but I don't see one.

A workaround is to give an initial assignment, but then we lose the ability to track whether any elements are not assigned -- for which we would still like to receive uninit errors. It would help if the Vector package exported a function that returns a Vector with uninitialized elements, and then we could replicate that to make a 2D Vector with uninitialized leaves. (Alternatively, if the Vector package defined and exported primMarkVectorInitialized, then we could call that on v -- as a workaround.)

A possible fix might be to define an instance of primMakeUninitialized for Vector n (Vector m a) that doesn't call markVectorUninitialized on the outer Vector. However, if the user failed to initialize one of the elements of the outer Vector, they would get m errors (for each of the elements of the sub Vector). It would be preferable to report just one error. So I'd like to find a better fix.

I assume that the error is because the assignment to v[i][j] is desugared into primUpdateFn applied to v[i], which therefore has to access v[i] that hasn't been assigned. Maybe we need a version of primUpdateFn that, instead of taking the new value of type t, takes a function t -> t that takes the current value as an argument -- and then we could nest calls to primUpdateFn.