Couldn't it already do the exact same thing without constexpr? (And shouldn't it have already done that when optimizing? In fact for simpler expressions compilers already do this, right?) How does specifying constexpr help?

But I think you nailed it -- the compiler doesn't have to signal optimization "failures" back to the developer, but it has to for the constexpr case. It is not that the constexpr routine can be used at compile time, it is that it must be useable at compile time.

> But I think you nailed it -- the compiler doesn't have to signal optimization "failures" back to the developer, but it has to for the constexpr case.

Is this true? Where do you see Clang emit a diagnostic in the example in the given article? (https://godbolt.org/g/HKcPFT)

You seem to be right -- at least "no diagnostics required" is mentioned a few times in $10.1.5 of N4700. To be honest, my comment is not from the article in the example, but from my own experience, and that is mostly with GCC 7.2.