Hello everyone.

A big thank you for the very insightful and quite long responses. I will try to respond to all of them, but this will take me some time.

Am 15.12.19 um 03:36 schrieb Arthur O'Dwyer:
On Sat, Dec 14, 2019 at 7:44 PM Martin Beeger via SG20 <sg20@lists.isocpp.org> wrote:
Concept will IMHO change how the think about template arguments and
template function radically, and will give us the power to express
semantic contracts much more clearly in definition.

Well, IMHO the opposite. :)  Ever since Stepanov in the mid-1990s, C++ programmers have known the general outlines of "generic programming": that in C++ it's based on templates, that templates impose syntactic and semantic requirements on their type parameters, and that a bundle of type requirements can be called a "concept."
C++11 allows us to write "concepts" fairly conveniently in the form of type-traits.
Orthogonally, C++98 allows us to write function templates that SFINAE away for types that don't match their preferred concept (in the form of `enable_if`).

I had a really good time reading "From Mathematics to Generic Programming" by Stepanov last year, which then motivated me to read most of "Elements of Programming" and although I somewhat knew about generic programming and Iterator concepts before, it really changed the way I think about writing templates. Armed with that knowlegde I tried to improve code quality by using enable_if as a concepts replacement. I first focussed on some key places, where to most lifting errors to compile time could be done. The results were, that it indeed catches error, but compile time explodes. It basically left us with the unfortunate choice of enable_if(ing) or code or compiling our code, but not both. I expect to be better off when Modules arrive, but until then, that in reality. And getting coworkers to think in terms of concepts when they are not at all enforced and completely invisible is hard.

C++2a adds new, shorter syntax to accomplish these tasks in fewer keystrokes, but it doesn't change how we should think about generic programming. The mere existence of shorter syntax should not lead us to use templates more often, nor to use SFINAE more often.

Interestingly Stepanovs book lead me already before concepts arrived to use templates less and localize them, by defining (or including from abseil) semantic types and program against valid semantic non-template types like duration instead of having templates and overload sets for different duration types (double seconds, int64_t ticks etc.).

But as one uses
concepts to modernize template functions, one quickly realizes that
nearly all template functions either have design flaws or have
constraints on template parameters which can be expressed by concepts.

Yes, there is no such thing as a "truly generic" template. A generic function does things to its arguments in terms of their capabilities (properties, affordances). A function that says "I compare my arguments" requires that its arguments be comparable (using the < syntax). A function that says "I swap my arguments" requires that its arguments be swappable (using some syntax). So, yes, all function templates impose (syntactic) requirements on their type parameters.
Any syntactic requirement can be expressed via C++2a Concepts, or via plain old C++11.
So, yes, every template imposes syntactic requirements on its type parameters which can be expressed by Concepts.
Equivalently, every template imposes syntactic requirements on its type parameters which can be expressed without Concepts.

If you have templates in your codebase today that are not SFINAE-friendly — that are not std::enable_if'ed on some type-trait that describes their syntactic requirements — then you should ask yourself, "Why?"  Why would a C++11 programmer occasionally write an "unconstrained" template, even as he knows that it must impose some kind of requirement and that the requirement must be expressible in C++?

As I said above: compile time. enable_if in about 20 core places doubles my compile time. type_traits and enable if template metaprogramming costs a lot.

Off the top of my head:
(1) The constraint is so complicated that it's not worth the effort to specify. For example, imagine the variadic version of this template:
    template<class F, class T1, class T2, class T3>
    bool all_permutations(F f, T1 t, T2 u, T3 v) {
        return f(t,u,v) && f(t,v,u) && f(u,t,v) && f(u,v,t) && f(v,t,u) && f(v,u,t);
It's probably possible to work out the C++ equivalent of "F is callable with all permutations of Ts...", but the cost/benefit analysis is against it.

This smells to me. You are doing something complicated, but general. So try to find an implementation elsewhere. Does the STL have it? Does abseil have it? Does hana have it? If not, is your original problem perhaps better seen in a different way? It may be the right thing to do, but sth. being a smell means for me "a reviewer stumbling upon should have some questions and some discussions" and "if that doesn't help, there should a comment explaining that you thought about it", not "you should never merge that".

Also I think the permutation stuff (at least very similar) is in Stepanovs book and I think he hard concepts for it (permutations are the basis of permuation-based sorting). So while I get your point, you might choose an example where Stepanov hasn't got you covered IIRC.

(2) The function does not need to mess with overload resolution, and it is so trivial that its type-requirement is no more or less than that its body compiles. For example:
    template<class T, class U> bool equals(T t, U u) { return t == u; }

This function is just a wrapper. You rename equality and inherit all the requirements. C++ hat no way to inherit/lift concepts, so yeah wrapper functions are a good answer to my questions. Your function requires copyability on top, but that can be fixed. There is a class of "semantically inactive" functions, like wrapper functions which themselves to not have concept requirements. I did not see that, thank you.

The error message "can't find a suitable operator== for t == u" is going to be just as useful to the client programmer as "can't find a suitable equals for equals(x, y)".
(3) The function does not need to mess with overload resolution, and for usability's sake, it prefers to static_assert its type-requirements rather than SFINAE away. For example:
    template<class Socket> void read_from_socket(Socket s) { static_assert(is_socket_v<Socket>); ... }

As for such a function the definition and declaration would usually seperate (as it is not a one-liner)  and if I program against it reading its declaration, I will use them wrong. Yes, the compiler shouts at me, which is good, but annoying. In a review I would question the usuability gain, looking a the actual usage of the function and other overloads.

(4) The compile-time cost of SFINAE is so large that the codebase has guidelines suggesting "no unnecessary SFINAE."  Notice that many templates are used as implementation details. For example:
    template<class Iter> void sort_helper(Iter a, Iter b, std::random_access_tag) { ... }
    template<class Iter> requires is_swappable_v<iterator_value_t<Iter>> void sort(Iter a, Iter b) { sort_helper(a, b, iterator_category_t<Iter>{}); }
I think most template programmers would agree that it would be counterproductive to repeat the same constraints on `sort_helper` that were already applied on `sort`. For one thing, it violates the Don't Repeat Yourself principle; but more importantly, it costs compile time and brings no benefit in return.

This is again a case of not repeating conecpts in semantically inactive wrapper functions, I get that. This specific wrapper function should no longer be needed with C++20, am I right?

This lead us to the conclusion, that one should have a coding guidelines
that flags unconstrained template parameters as a code smell.

I think this is viable for small codebases, but I suspect a significant fraction of large codebases would run into #4 above if they tried to add constraints on all their templates.

A significantly large codebase (where "significantly" is up for debate) can only afford a limited number of template function instantiations. Otherwise its compile time will blow up anyway. I would rather afford a few less template functions, with well defined named concepts instead of a larger number of template function w/o them. Hopefully modules can alleviate some of this tension.

We really struggled to find one by looking though our codebase. The plus 
function came to mind, but this requires the arguments to form a magma
at least, which is definitively a valid requires clause.

The plus function obviously requires `t + t` to be well-formed, which is a valid requires-clause.
FWIW, I dispute your claim that this has anything to do with mathematical magmas. A magma is a set (i.e. a type) with a binary operation (let's spell it `operator+`) which is closed; I interpret that in a C++ context to mean that decltype(t+t) ought to be T, or at least something explicitly convertible to T. But that's definitely not a requirement imposed by the `plus` function! Not if it's defined as
    template<class T> auto plus(T t, T u) { return t + u; }
 anyway. However, I suggest we don't pursue this rabbit-hole. We can pursue the next one if you like. :)

Oh yeah, there is a deep rabbit hole about whether is is benefical to have plus over a closed set separate from plus over an open set. And how to express commutativity here... Lets not go there.

The next
candidate was the identity function, but for the identity function to be
correct, its result must be identical to the input, which only has valid
meaning if the input and the output is actually equality comparable,
which is a valid requires clause.

Here I absolutely disagree. The `identity` function
    template<class T> T identity(T t) { return t; }
requires that T be move-constructible and destructible, but absolutely does not require that T be equality-comparable. For example, `identity` remains a useful operation even when T is `std::function<void()>`. `std::function<void()>` is not, and cannot ever be made, equality-comparable.

The type-requirements of a generic algorithm are driven by what the algorithm actually does. It requires what it does to be well-formed. My kneejerk impression is that no generic algorithm should ever "require" some capability of its type parameter if its implementation does not depend on that capability.
But for a non-comparable type template<class T> T identity(T t) { return T {}; } is often a faster implementation. It may not be what the programmer expected, but how to write test? Equality is a postcondition of the type. As the postcondition is impossible to verify, semantics meaning of the function gets lost and the function itself gets misleading. If you wanted a "semantically inactive" no-op function, state that an name it appropiuately. Identity has a mathematical meaning that different from no-op. I think the identity function on non-comparable types would not be "provable" in Lisa Lippincott's sense. So I personally claim it should not be written that way.
To take a really extreme and probably singular example, imagine what the STL would have been like if we'd said, "The std::copy algorithm requires the expression `std::addressof(*dest)` to have pointer type." That's "obvious," right, because the point of std::copy is to be a type-safe replacement for memcpy, and objects always have addresses? ...But then we never would have gotten ostream_iterator or back_insert_iterator!

The copy function is defined without mention of memcpy. A copy function is a function which takes Range T and produces a new range T2 where the following two postconditions hold: The range T is unchanged and the range is T2 equal to the original range T. And if adressof(*dest) is not a pointer, the function can still reach its postconditions. So this is not really a requirement. memcpy-ability or relocatability (yes I have seen your talk :)) how you call it, may be a requirement of candidate within the overload set of std::copy to select the fastest algorithm but never a condition on the copy overload set as a whole, because such a set I would call copy_to_pointer_location.

I suspect that a sufficiently motivated programmer could always find a semantic requirement that you failed to capture in your naming scheme. ;) Even something as simple as "fclose(fp) expects fp to be a file that is open, not a file that has already been closed."  But maybe it's domain-dependent; maybe "no hidden semantic requirements" actually works in your particular codebase.

I agree. This is a Sean-Parent-esk "goal". You will never reach it, but dude, how much cool stuff you can get from trying and even getting part of the way there.

If fp was an RAII object which invariant was: for the object to exist, a open file must exist, the semantic requirement of fclose would be to get a valid FP object of which its invariant is satifisfied, et voila, you are done. You cannot correct missing invariant on the type definition of a function parameter at the function declaration. Get your types right and RAII for the win.

preferred over auto, contains a semantically meaningless auto! A rule
along the lines of "just avoid auto" is much simpler than to teach than
a rule along the lines of "just avoid auto, except when there is this
confusing thing which spells auto but is not really a semantic auto, as
it will not match what auto normally matches". So a a result, this
shorthand notion tends to be avoided out of misinterpretation of the
guidelines, which then makes concepts unnecessarily hard to use.

I do think it's important to teach that (as of C++14) there are two different possible meanings for "auto" depending on where it appears in the program. When I write

    auto int f() { auto i = 0; for (auto&& elt : vec) ++elt; return i; }
    template<class T> void g() { auto x = T(); }

all four of those "auto"s mean the same thing: "There is one single concrete type that I have in mind, and I am just too lazy to write it out. Compiler, please infer it for me." (In the last case, the concrete type differs in each specific template instantiation, but it's always the same as `T`.)
In contrast, when I write

    std::sort(first, last, [&](auto&& a, auto&& b) { return a < b; });
or in C++2a
    void h(auto x) { static int y = 0; std::cout << x << ++y; }

there the "auto"s mean something vastly different: "This thing is a template. There is no one concrete type I have in mind; please generate a template that can be instantiated for many different things."
Importantly, since `h` is a template, there are many different `y`s — one per instantiation — not just one `y`.

I will go into more detail in the response to Herb Sutters mail, which also covers this.

For this reason, I conservatively suggest avoiding the `void func(Iterator auto it)` syntax that you dislike, and preferring an old-fashioned
    template<Iterator It>
    void func(It it) { ... }
or even
    template<class It> requires Iterator<It>
    void func(It it) { ... }
which makes it clear that you want a template and you want it to SFINAE away when the parameter type wouldn't be an Iterator.
This is worth some experimentation. Thank you.

I can understand that a syntax of "void func(ForwardIterator it)" raised
some eyebrows, but what is the reasoning behind preferring a syntax of
"void func(ForwardIterator auto it)" over alternatives like "void
func(concept ForwardIterator it)" or similar? Or am I missing a way to
consistently tell this story and not confuse people?

I don't think `void func(concept ForwardIterator it)` was ever proposed.
("If you didn't want the product to be bad then why didn't you write a paper proposing that the product be good?" ;))
I value understanding the current situation before one tries to change it. So I deemed a mail to a mailing list a better start than a paper. And as I found the misunderstanding to be on my end, this was the right choice.
However, `concept Foo` is also more typing than `Foo auto`, so I don't think it's an improvement.
Right now, `auto x` in a lambda parameter list means "secretly a template," so it was the obvious choice when secret templates were extended to work with plain old functions.  It was a GCC extension long before it was in C++2a.
(In fact, just last week I had to explain to an actual beginner student that one of his C++14 program's `auto`s was legit and the other was exploiting a non-standard GCC extension! Stupid GCC...)
Herbs mail told me what I am missing and my lack of knowledge over the final version of concepts did to lead to that confusion on my end. Sorry.

Has this been talked about or what is the general plan to get our
community to move away from unconstrained template parameters and auto?

- Whose community (yours in particular, or postulating some global C++ community)?
- Should any community move away from unconstrained template parameters, and if so, why?

If you don't like the visual similarity of
    void foo(Iterator auto x)  // TWO WORDS GOOD
    void foo(auto x)  // ONE WORD BAD
then what would you say to
    template<Iterator It> void foo(It x)  // NO CLASS GOOD
    template<class It> void foo(It x)  // CLASS BAD

I notice that `template<class It> void foo(It x)` doesn't contain the dreaded word "auto", but presumably you'd still consider it a "bad" unconstrained template, right?  So it's not exactly the word "auto" that's causing you trouble...?

The game-changer here is actually the simple line in Herbs mail:

iterator auto first = vec.begin()

I did not know that concepts could also be used to constrain auto in immediately deduced concept like statements, not only in later instantiated template contexts. And as both were called auto, this should have been somewhat obvious to me but it wasn't. In my mind, concepts could only be used to constrain templates, and I never thought of auto it= vec.begin(); as a template, but as a deduced type. So I did not expect you to be able to constrain it, and worried about the task to disambigaute between these two kinds.

Now everything is beautiful: The story to tell is just use auto, but when you do and you have a named concept in mind and at hand, constrain your auto by using the concept. You do that, by simply writing the constraint before the auto. If that works for all auto contexts and no asterisk is needed, this is really beautiful.