[llvm-bugs] [Bug 51918] New: [concepts] improve diagnostics for range concept failures

[via llvm-bugs]

https://bugs.llvm.org/show_bug.cgi?id=51918

            Bug ID: 51918
           Summary: [concepts] improve diagnostics for range concept
                    failures
           Product: clang
           Version: trunk
          Hardware: PC
                OS: All
            Status: NEW
          Severity: enhancement
          Priority: P
         Component: C++2b
          Assignee: unassignedclangbugs at nondot.org
          Reporter: ldalessandro at gmail.com
                CC: blitzrakete at gmail.com, erik.pilkington at gmail.com,
                    llvm-bugs at lists.llvm.org, richard-llvm at metafoo.co.uk

Clang produces some concepts diagnostics that are of limited use to the
programmer, particularly with respect to the ranges library.

In this case, I was playing around with proxy iterators and ran into
https://godbolt.org/z/e9M9T3z5j, which tells me that my iterator type fails the
borrowed_iterator_t requirement, however it gives me absolutely no information
about _why_. 

The borrowed_iterator_t is not a trivial concept, when I google for it I get
https://en.cppreference.com/w/cpp/ranges/borrowed_iterator_t which is split
into two cases as:

    1) std::ranges::iterator_t<R> if R models borrowed_range,
std::ranges::dangling otherwise.
    2) std::ranges::subrange<std::ranges::iterator_t<R>> if R models
borrowed_range, std::ranges::dangling otherwise.

As a user I have no idea what to do at this point.

The reduced code I was looking at was

```
struct Broken {
    struct iterator {
        int i;
        auto operator*() const -> decltype(auto) {
            return std::tie(i);
        }
        auto operator++() -> decltype(auto) {
            ++i;
            return *this;
        }
        friend bool operator==(iterator const&, iterator const&) = default;
        friend auto operator<=>(iterator const&, iterator const&) = default;
    };

    auto begin() const -> iterator { return {}; }
    auto end() const -> iterator { return {}; }
};

int foo() {
    Broken r;
    std::ranges::for_each(r, [](auto t) { 
        auto [i] = t;
        printf("%d\n", i);
    });
}
```

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