JS: adding to uninitialized seq/string inside object variant is no-op

[metagn]

Example

type
  A = object
    case kind: bool
    of true:
      sons: seq[int]
    else: discard

var a = A(kind: true)
echo a.sons.len
a.sons.add(1)
echo a.sons.len

Current Output

0
0

Expected Output

0
1

Additional Information

• This breaks parsesql for JS (with StringStream & more stdlib modules support for JS/NimScript #14095) and probably way more standard library modules that I haven't tested yet.

$ nim -v
Nim Compiler Version 1.2.0

[metagn]

Codegen for a.sons.add(1):

var Tmp1;
var Tmp2;
var a_622008 = [{kind: true, sons: null}];
var Tmp1 = a_622008[0];
if (ConstSet1[Tmp1.kind]===undefined) { raiseFieldError(makeNimstrLit("\'sons\' is not accessible using discriminant \'kind\' of type \'A\'")); }
if ((Tmp2 = Tmp1.sons, Tmp2) != null) { Tmp2.push(1); } else { Tmp2 = [1]; };

Codegen if you remove case branch in the object:

var a_622007 = [{sons: null}];
if (a_622007[0].sons != null) { a_622007[0].sons.push(1); } else { a_622007[0].sons = [1]; };