Besides being deprecated in C++11 and removed in C++17, the intended use
case for this was always wrong, since std::unexpected() is called by the
C++ runtime when a function throws an exception that was not specified
in its "dynamic exception specification", which is different from an
exception thrown by user code which wasn't caught [1,2]. Using abort()
keeps the same behavior, but with the intended semantics.
We don't use std::abort() to simplify backwards compatibility.
[1] https://github.com/epics-base/epics-base/issues/343
[2] https://en.cppreference.com/w/cpp/error/unexpected
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