JSON reader misreads a complex value inside an array of tuples

[jgabry]

If there's a complex number nested in a tuple that is itself in an array, the JSON reader reads it incorrectly. The same tuple outside an array is handled fine.

data {
  array[2] tuple(real, complex) d;
  array[2] tuple(complex, real) e;
  tuple(real, complex) f;
}
generated quantities {
  real d11 = d[1].1;
  real d12r = get_real(d[1].2);
  real d12i = get_imag(d[1].2);
  real d21 = d[2].1;
  real d22r = get_real(d[2].2);
  real d22i = get_imag(d[2].2);
  real e11r = get_real(e[1].1);
  real e11i = get_imag(e[1].1);
  real e12 = e[1].2;
  real e21r = get_real(e[2].1);
  real e21i = get_imag(e[2].1);
  real e22 = e[2].2;
  real f1 = f.1;
  real f2r = get_real(f.2);
  real f2i = get_imag(f.2);
}

{"d":[{"1":1.0,"2":[2.0,3.0]},{"1":4.0,"2":[5.0,6.0]}],
 "e":[{"1":[7.0,8.0],"2":9.0},{"1":[10.0,11.0],"2":12.0}],
 "f":{"1":13.0,"2":[14.0,15.0]}}

Run with method=sample algorithm=fixed_param num_samples=1.

I noticed this when working on better support for complex numbers and tuples in cmdstanr. I asked Claude to make this table (it also helped generate the Stan code above):

value	expected	read
d12r, d12i	2, 3	2, 5
d22r, d22i	5, 6	2.2e-314, 2.2e-314
e11r, e11i	7, 8	7, 10
e21r, e21i	10, 11	2.2e-314, 2.2e-314

The real parts of both elements end up in the first complex value and the second is just uninitialized. The tuple's real element (d11, d21, e12, e22) and the non-array tuple f seem OK.

[jgabry]

This is CmdStan 2.40.0 on my MacBook Pro

[WardBrian]

This was actually a miscompile in stanc, I was too hasty moving it to stan. PR is up stan-dev/stanc3#1734

[WardBrian]

Actually, I think it's both a miscompile in stanc (leading to the uninitialized read) and a bug in the JSON reader here (which leads to the wrong value being used, even after fixing the uninitialized read)