Comment by demorro

I know you were simplifying, but "does foo then bar" is so far away from what an actual specification is that it defeats the point.

A more complete spec will capture performance requirements, input preconditions and output postconditions, error handling and recovery behaviors, threading behaviours, hardware assumptions, etc. It's hard to do these things without leaning at least somewhat on the specific language runtime you are using, otherwise you'd end up regurgitating the C standard each time you design a software system.

It's this sort of stuff that is meant when people say "sufficiently detailed".

If you're actually testing all these things, then I might agree with you that you can do it in the tests, but almost no one actually is. I'd struggle to write a test suite that tests all the specification-level assumptions I draw from my language and target platforms.