Bug in HVM compiler: affinity is not checked

[adamritter]

This program compiles to an invalid interaction net and works:

@double(a) =  (+ a a)
@main = @double(3)

The compiled output creates 2 references to the passed variable:

Term double_t(Term ref) {
  Loc opx0 = alloc_node(2);
  set(opx0 + 0, got(term_loc(ref) + 0));
  set(opx0 + 1, got(term_loc(ref) + 0));
  return term_new(OPX, 0, opx0);
}

In the current case main is 6, so it seems like the program is working, but more complex programs have serious issues as the compiled code gets incorrect.

The compiler SHOULD check if a variable is used twice and compilation SHOULD fail.