runtime error with clang sanitizer

[coin-au-carre]

The option -fsanitize=integer with Clang 3.7.1 gives several warnings of the type :

docopt_util.h:96:34: runtime error: unsigned integer overflow: 6142509191626859559 + 15635260900501029888 cannot be represented in type 'unsigned long'
SUMMARY: AddressSanitizer: undefined-behavior

[jaredgrubb]

Unsigned overflow is well-defined as wrap-around. In this code, overflow is expected and is ok. Maybe adding an explicit cast to say "this will overflow and that's ok" would quiet the warning.