assigned to @HazardyKnusperkeks

Ditto Poke, poke. Now that C++20 has been sent out for (what will likely be) its final round of balloting, it would be a good time to work on supporting requires-clauses in clang-format. Think of all the poor, suffering STL maintainers!

I've started to look into the concepts and requires keywords and how clang-format interacts with them (its doesn't really)

I'd like @​Casey to start with a slightly simpler example (if we could start without the comment that might help)

For the following would you expect it to be formatted like this?

template <class I, class S>
requires Sentinel<S, I>()
constexpr void advance(I &i, S bound) noexcept( noexcept(++i != bound)) {
while (i != bound) {
++i;
}
}

or something else? I'm trying to look for something of a common starting place between what I see in both

https://github.com/CaseyCarter/cmcstl2
and
https://github.com/microsoft/STL

I've started to look into the concepts and requires keywords and how
clang-format interacts with them (its doesn't really)

I'd like @​Casey to start with a slightly simpler example (if we could start
without the comment that might help)

For the following would you expect it to be formatted like this?

template <class I, class S>
requires Sentinel<S, I>()
constexpr void advance(I &i, S bound) noexcept( noexcept(++i != bound)) {
while (i != bound) {
++i;
}
}

Today, I'd format this as:

  template <class I, class S>
    requires Sentinel<S, I>()
  constexpr void advance(I& i, S bound) noexcept(noexcept(++i != bound)) {
    while (i != bound) {
      ++i;
    }
  }

with the requires-clause indented once relative to the template declaration, and continuation lines one indent deeper:

  template <class I, class S>
    requires Sentinel<S, I>() && // ...
      && OtherConcept1<I> && // ...
      && OtherConcept2<I> && // ...
      && OtherConcept3<I> && // ...
  constexpr void advance(I& i, S bound) noexcept(noexcept(++i != bound)) {
    while (i != bound) {
      ++i;
    }
  }

which probably follows from how clang normally wraps expressions across lines. I try to format trailing requires-clauses exactly as you've put the noexcept-specifier in the example, but wrapping the whole thing onto the next line if it won't fit on the primary declaration line and only splitting if the requires-clause itself is longer than one line:

  template <class I>
  void f(I i) requires Concept1<I> && Concept2<i> {
    // ...
  }

  template <class I>
  long_return_type g(I i) 
    requires Concept1<I> && Concept2<i> && VeryVeryVeryVeryLongConcept<I> {
    // ...
  }

  template <class I>
  long_return_type h(I i) requires Concept1<I> && Concept2<i> && 
    VeryVeryVeryVeryLongConcept<I> && OtherLongConcept<I> {
    // ...
  }

or something else? I'm trying to look for something of a common starting
place between what I see in both

https://github.com/CaseyCarter/cmcstl2
and
https://github.com/microsoft/STL

Please ignore cmcstl2: there a few different styles there that we experimented with over the years, and it was never really made consistent. range-v3 is even worse: its style represents a set of compromises to get the least offensive output from clang-format. I have no idea how libstdc++ is formatting their concepts usage, but they have C++20 Ranges implemented so they may have an interesting counterpoint to my style.

I'm working on it.

The review link would be: https://reviews.llvm.org/D113319
But I have decided that the style

template <typename T>
requires
         C<T>
class ...

will not be supported. If you are really interested in it you can look at some older revision where I had a basic handling of that and try to get it working.

Please update this, either as solved, or what is still wished and we can discuss that.

Let's go ahead and call this closed. Anyone who wants changes can file a new issue.