* Provide the programmer control over every aspect of performance: Compile-time vs run-time execution is an aspect of performance.
* Excellent ergonomic: Using constexpr/consteval improves on earlier mechanisms for achieving compile-time evaluation, such as macros, template meta-programming etc. Those are unwieldy, often difficult to read, and inflexible (without jumping through hoops). constexpr is not.
etc. etc.
and marginally supports the following goals:
* Code should perform predictably: Control over what exactly runs at run-time improves predictability.
* Leave no room for a lower level language: While constexpr is neither low nor high as a language feature, one can argue that having it undercuts the potential for, say, a "C + constexpr"-like language, or another lower-level language with a more elaborate macro system.
etc. etc.
I could go on naming goals constexpr supports, there are lots more. I challenge you to find goals from which it detracts.
Not only was P2137 "not formally adopted" it was purposefully written so that the committee could say explicitly "No" to these goals. It's in some sense a manifesto for one of the 2022 "C++ successor" languages, Carbon not for C++.
If you want to cite actual goals, you won't get much from WG21, which prefers not to get tied down by having any principles to speak of. Bjarne himself however wrote a book in 1994 "Design & Evolution of C++" which might help you determine some goals.
I don't see constexpr functions in particular as a success. There are too many caveats, it's easy to find C++ programmers who've written what they assume is a constant evaluated expression but it isn't, for one of many reasons their compiler has decided it's runtime evaluated, and the programmer was surprised.
The reason the standard doesn't guarantee constant evaluation of constexpr is that it would be hard, because of the as-if rule, to specify it in an good way other than a non-normative comment. Remember that as long as the sequence of side effects is preserved (and even that, concurrency introduce non-determinism), any translation is compliant (even no translation in case of interpretation).
In practice you do get guaranteed evaluation wherever the result is required at compile time (for example as a non-type template parameter or as an array size).
Your first point is important, and now that you mention it, I do remember similar names from the Carbon introduction and this paper. I should probably look to D&E for a less presumptive set of goals.
As for constexpr - you only get ambiguity if you assign a constexpr expression/function call to a non-constexpr value. If you use constexpr variables to hold the results of your computation, compile-time evaluation is guaranteed. I don't think it's that confusing.
I wrote that constexpr is useful. In fact that is just my point, a useful feature brings controverse. My take on it is that it is because the language is f up.
constexpr, amongst other uses, cleans up code that was hard-to-understand templated code.
Static assert cleans up other complicated constructs.
Make_unique together with heaps of other primitives tries to clean up cumbersome memory management.
[[fallthrough]] tries to solve the fact that break is easily forgotten, but this time not forgotten, but explicitly not there...
You can name so many of new constructs that solve an earlier problem.
If you went through all the cpp standards upgrades, and knew the history, you understand why all these improvements exist. If not, it all looks overly complicated, which in fact, if you look at it objectively, it is.
I believe this misconception on your part stems from a lack of awareness (or understanding) of C++ design goals as a language.
Have a look at this writeup: https://www.open-std.org/jtc1/sc22/wg21/docs/papers/2020/p21... (caveat: not formally adopted)
constexpr supports the following goals:
* Provide the programmer control over every aspect of performance: Compile-time vs run-time execution is an aspect of performance.
* Excellent ergonomic: Using constexpr/consteval improves on earlier mechanisms for achieving compile-time evaluation, such as macros, template meta-programming etc. Those are unwieldy, often difficult to read, and inflexible (without jumping through hoops). constexpr is not.
etc. etc.
and marginally supports the following goals:
* Code should perform predictably: Control over what exactly runs at run-time improves predictability.
* Leave no room for a lower level language: While constexpr is neither low nor high as a language feature, one can argue that having it undercuts the potential for, say, a "C + constexpr"-like language, or another lower-level language with a more elaborate macro system.
etc. etc.
I could go on naming goals constexpr supports, there are lots more. I challenge you to find goals from which it detracts.