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Certain things, when used in a routine, make computation impossible at compile time. If the routine is marked with 'constexpr', the compiler will verify that.

• mehrdadn 9y ago
  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?
  • minipci1321 9y ago
    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.
    • mehrdadn 9y ago
      > 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 https://godbolt.org/g/HKcPFT)
      • minipci1321 9y ago
        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.