This is not pure C++.
It looks like somebody has written either a compiler extension or a series of macros to allow the specification of pre- and post-conditions for a block of code.
You’re going to have to ask that person.
(As an aside, this is why just “extracting code” is often insufficient for understanding; you need to observe the full context in order to know what’s going on. In this case, the context includes definitions and/or documentation found elsewhere and not included in your question.)
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solved What is different between typedef with invariant and nomal typedef [closed]