Program abort under EmbeddedVector flattening

[rcolyer]

This bug was initially raised on the OpenSCAD mailing list http://forum.openscad.org/RC-dumps-core-tc31122.html as a regression in the master branch of OpenSCAD for the BOSL2 library's testing suite test_hull.scad https://github.com/revarbat/BOSL2/blob/master/tests/test_hull.scad

Extensive reduction and transformation of the code in question reduced it to the following test case:

function nop(x) = 0;

function fails() =
  let (
    arr = [for (c = [0,0], i = [0,0]) 0],
    x = [for (e = arr) nop(arr[0])]
  ) 0;

function save_by_flattening() =
  let (
    arr = [for (c = [0,0], i = [0,0]) 0],
    tmp = arr[0],
    x = [for (e = arr) nop(arr[0])]
  ) 0;

function works() =
  let (
    arr = [for (c = [0,0,0,0])        0],
    x = [for (e = arr) nop(arr[0])]
  ) 0;

x = fails();
//x = save_by_flattening();
//x = works();

Running as:

openscad -o - --export-format echo modified_test.scad

produces an error output of:

openscad: src/value.cc:211: Value Value::clone() const: Assertion `false && "unknown Value variant type"' failed.
Aborted

After a detailed interpretation discussion on IRC with @thehans. This issue was diagnosed as most likely due to an invalidation of EmbeddedVector iterators during flattening, which causes undefined behavior leading to a faulty pointer to a Value with an unknown variant type.