[masklinn]

> It's such a stupid idea to require an annotation everywhere you want to evaluate things at compile time, practically everything will inevitably be evaluatable at compile time

And making constexpr-ness an explicit contract makes sense to me: if it's not that it can be an unexpected property, and can break at any change of implementation.

Yes requiring a function being marked requires that the implementor do it, but it also means they have considered this use-case and made it officially part of the API. It's not a trivial promise to make.

> and you need the implementation anyway, so just let it fail rather than ask for permission everywhere.

"Let it fail" is the issue, if it's implicit a user can assume this is working by design, then find their program stops compiling on the next release not because the maintainer wilfully broke the API, but because they changed an implementation detail of the function and constexpr-ness is not something they considered (or assumed correct) in the first place.

Maybe the language should work the other way around and everything should be constexpr by default and functions should opt out of constexpr-ness, but that's 40 years too late for C++. And I can't think of any langage which does that. And frankly it feels like the wrong default for the reasons above.

[Jasper_]

Except constexpr isn't a guarantee. Compilers can choose to silently evaluate constexpr things at runtime. And I have ran into this before: a compiler with a recursion limit causing it to bail on constexpr and just emit the code.

So constexpr isn't a guarantee of it being evaluated at compile time, and non-constexpr isn't a guarantee of it being evaluated at runtime. Cool, huh?

[jcelerier]

> constexpr isn't a guarantee of it being evaluated at compile time

constexpr is a guarantee that you can use the thing in a constexpr context, and this is where the "evaluated at compile-time" guarantee can come from:

    template<typename T>
    auto func() {
      // here some compilers can still choose to evaluate x at run-time - and very likely all of them if no optimizations are enabled
      constexpr int x = f(); 

      // but here it becomes mandatory for this use of x to be evaluated at compile-time, since the number is literally going to be part of the compiled binary as part of the function name mangling
      return std::integral_constant<int, x>{};
    }

[gpderetta]

Constexpr does not guarantee that a constexpr function can be called at compile time, unfortunately. Only that it can be called at runtime for a subset of all possible parameters.

Except of course the subset of parameters for which it is constexpr callable can't be checked at function definition time (if it exists at all), only at function invocation time.

Which makes the constexpr annotation useless and it is in only because the authors couldn't otherwise get the paper through some committee objections.

[Dylan16807]

Is such a mangling mandated by the standard?

[mhh__]

Mangling is mentioned in the standard.

In this case though the underlying reason is that its part of the type (system) not because of the mangle specifically.

[Dylan16807]

> Mangling is mentioned in the standard.

Forgive me, but can you be clearer than "mentioned"? Is the mangling required to contain template parameters for return types?

> In this case though the underlying reason is that its part of the type (system) not because of the mangle specifically.

I'm not sure. The compiler knows it will always be the same type, so under many uses of this function I could easily imagine a compiler that doesn't actually fill in .value until runtime.

[jcelerier]

> Forgive me, but can you be clearer than "mentioned"? Is the mangling required to contain template parameters for return types?

The mangling will contain template parameters, as you can have:

foo.hpp

    template<typename T>
    T f();

foo.cpp

    template<>
    int f<int>() { return 123; }
    template<>
    float f<float>() { return 123; }

bar.cpp

    std::cout << f<int>();

and the right function has to be found. demo: https://gcc.godbolt.org/z/rMjYoEzaK

[mhh__]

Mangling isn't technically required as per se but it's by far the most common approach (basically a local minimum in terms of cost)

I think this subset of C++ templates is probably undecidable still.

[masklinn]

Mostly makes sense to me: constexpr means the function is evaluatable at compile-time, being callable at runtime is part of the contract and why adding constexpr does not change the API.

For an assertion that something is evaluated at compile time I’d assume a variable-level annotation (in a non-constexpr function). Though I don’t know c++ enough to have any idea whether that’s the case.

And contant evaluation optimisation has always been a thing so it’s not really surprising.

[Dylan16807]

> Mostly makes sense to me: constexpr means the function is evaluatable at compile-time, being callable at runtime is part of the contract and why adding constexpr does not change the API.

The problem isn't that you're able to call it at runtime with runtime data, it's that if you give it compile-time data you have no idea when it will be run.

> For an assertion that something is evaluated at compile time I’d assume a variable-level annotation (in a non-constexpr function). Though I don’t know c++ enough to have any idea whether that’s the case.

Oh, were you under the impression that constexpr was just for functions? It applies to variables too, and it's not a guarantee on them. You need to use other, newer annotations.

[comex]

C++20 has consteval and constinit to fix this (at the cost of making things even more complicated).

[MrBuddyCasino]

Amazing.

[WalterBright]

> I can't think of any langage which does that

D does. And it's a very popular feature. In fact, D goes even further - only the path taken through a function needs to conform when running it at compile time, not 100% of the function.

> it feels like the wrong default for the reasons above.

In about 16 years of very extensive use, it has never been brought up as a problem.

[throwaway173738]

The other problem with implicit constexpr is that it can change the timing characteristics of your code depending on whether your input data is known at compile time. And then if you want it to be constexpr you have to troubleshoot why it sometimes isn’t. I think this approach of maximal control in exchange for some foot guns is very much in keeping with C++.

[WalterBright]

In D you don't have to troubleshoot it. The compiler will tell you where the problem is if it can't be evaluated at compile time.

> for some foot guns

There just aren't any with this feature.

[scatters]

> Maybe the language should work the other way around and everything should be constexpr by default and functions should opt out of constexpr-ness, but that's 40 years too late for C++.

That's how lambdas behave in C++, so it's definitely feasible. gcc has a flag, -fimplicit-constexpr I think, that does this for regular functions, and it doesn't appear to cause any significant issues. I think there has been some talk around making this the language behavior at some point in the future.

[mhh__]

> It's not a trivial promise to make.

It's actually very trivial. 99% of code I write can be run at compile time.

> maintainer wilfully broke the API

Did they break the API or did they change the semantics? The API is tittle-tattle, the point is that you cannot run it at compile time anymore (e.g. accidentally introduced a opaque new dependency by accident? Oops)

> And I can't think of any langage which does that

Being able to opt out is not what you want, but D allows everything to at least attempt to be run at compile time. This has been very profitable.

[masklinn]

> 99% of code I write can be run at compile time.

Good for you, you missed the point.

> Did they break the API or did they change the semantics?

What are you even on about?

> The API is tittle-tattle

What are you even on about, part 2.

> the point is that you cannot run it at compile time anymore

Yes, the API allowed one thing, now it does not. That's no different from changing an optional parameter to mandatory or any other thing which breaks the API.

[mhh__]

> Good for you, you missed the point.

>> Did they break the API or did they change the semantics?

> What are you even on about?

Constructive.