The weakness of C++ contracts is that they are mostly syntactic sugar over C asserts.
The sugar is actually one of my issues with them. The evaluation semantics are much more complicated / potentially slower than C asserts
because of the sugar, and why you would have to pay those costs isn't obvious at first glance. Lisa Lippincott's excellent talk on the design of contracts [0] is what switched me from a sort of neutral support to disliking it. I think it's a stupid place for the national bodies to draw the line though. std::regex is acceptable, but a nothingburger like contracts tanks the vote?
[0] https://www.youtube.com/watch?v=yhhSW-FSWkE
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To be clear, I have a lot of issues with the contracts proposal. I probably wouldn’t use despite living out on the standards frontier. I don’t even use C asserts already because they are not expressive enough in important ways. This proposal kind of reifies the inadequacies.
I think the risk is that C++ contracts become an evolutionary dead-end that hang around forever in code bases because they were standardized. It wouldn’t be the first time this has happened.