Several researchers have recently implemented on-line testability in the form of duplication-based self-checking digital system design, early in the design process. We consider on-line testability within the optimization phase of iterative, cost function-driven high-level synthesis, such that self-checking resources are inserted automatically without any modification of the source behavioral HDL code. This is enabled by introducing a metric for on-line testability. A new variation of duplication (namely inversion testing) is also proposed and used, providing the system with an additional degree of freedom for minimizing hardware overheads associated with test resource insertion. Considering on-line testability within the synthesis process facilitates fast and painless design space exploration, resulting in a versatile high-level synthesis process, capable of producing alternative realizations according to the designer’s directions, for alternative target technologies. Finally, the fault escape probability of the overall scheme is discussed theoretically and evaluated experimentally.