Would you recommend a different strategy for libraries, say, than apps?
Wouldn't you want a given library version to be totally covered?
I don't think that more coverage is a bad thing, especially with something that is a library, and thus is going to be used by multiple applications. One of the key takeaways to all of this is that if we say that "totally covered" is 100%, and that we enforce 100%, you're going to get some unwanted results.
I don't think it's bad to strive for 100%, it's a common occurrence with TDD which I believe in.
I think it's bad when you say all apps must have 100% and we'll even fail the build at 100%. There are going to be cases in which 100% doesn't make sense, so developers will have to get cute to make the build pass.
Put as much testing effort as you believe makes sense. If it's a library, go nuts. Maybe you'll get 100%, maybe you'll get 90%. What matters is that you made deliberate decisions to test what needs to be tested, and the coverage % is a reflection of that.
Then, going forward, you can monitor coverage as a metric. You can see if coverage going down, and if it is, ask yourself or your team why.
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