GSL should obey the core guidelines. Expects and Ensures should be GSL_EXPECTS and GSL_ENSURES. As the person who has suffered for 30 years for defining widely used macros check and require I can attest to just how bad an idea those names are. (https://opensource.apple.com/source/xnu/xnu-7195.141.2/EXTERNAL_HEADERS/AssertMacros.h.auto.html)