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It's very natural to want to compare std::array's at compile time; and its operator==() is obviously constexpr'able. Yet - it isn't marked constexpr. Is this intentional or an oversight? And - what's the reason it was left that way (apparently in C++17 as well)?
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Constexpr constructors are permitted for classes that aren't literal types. For example, the default constructor of std::unique_ptr is constexpr, allowing constant initialization.
constexpr std::string While it's best to rely on string_views and not create unnecessary string copies, the example above shows that you can even create pass vectors of strings inside a constexpr function!
P0031 explained why it didn't propose constexpr comparisons:
Currently comparisons and
swap/fillmay be implemented with the help of algorithms from<algorithm>header. Marking comparisons with constexpr will break that ability and will potentially lead to performance degradations.
For example, == can be implemented in terms of std::equal, which - in appropriate cases - can call the highly-optimized-but-decidedly-not-constexpr memcmp. Making constexpr for == will rule out this optimization without special compiler assistance.
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