<html>
<head>
<base href="https://bugs.llvm.org/">
</head>
<body><table border="1" cellspacing="0" cellpadding="8">
<tr>
<th>Bug ID</th>
<td><a class="bz_bug_link
bz_status_NEW "
title="NEW - [concepts] improve diagnostics for range concept failures"
href="https://bugs.llvm.org/show_bug.cgi?id=51918">51918</a>
</td>
</tr>
<tr>
<th>Summary</th>
<td>[concepts] improve diagnostics for range concept failures
</td>
</tr>
<tr>
<th>Product</th>
<td>clang
</td>
</tr>
<tr>
<th>Version</th>
<td>trunk
</td>
</tr>
<tr>
<th>Hardware</th>
<td>PC
</td>
</tr>
<tr>
<th>OS</th>
<td>All
</td>
</tr>
<tr>
<th>Status</th>
<td>NEW
</td>
</tr>
<tr>
<th>Severity</th>
<td>enhancement
</td>
</tr>
<tr>
<th>Priority</th>
<td>P
</td>
</tr>
<tr>
<th>Component</th>
<td>C++2b
</td>
</tr>
<tr>
<th>Assignee</th>
<td>unassignedclangbugs@nondot.org
</td>
</tr>
<tr>
<th>Reporter</th>
<td>ldalessandro@gmail.com
</td>
</tr>
<tr>
<th>CC</th>
<td>blitzrakete@gmail.com, erik.pilkington@gmail.com, llvm-bugs@lists.llvm.org, richard-llvm@metafoo.co.uk
</td>
</tr></table>
<p>
<div>
<pre>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
<a href="https://godbolt.org/z/e9M9T3z5j">https://godbolt.org/z/e9M9T3z5j</a>, 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
<a href="https://en.cppreference.com/w/cpp/ranges/borrowed_iterator_t">https://en.cppreference.com/w/cpp/ranges/borrowed_iterator_t</a> 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);
});
}
```</pre>
</div>
</p>
<hr>
<span>You are receiving this mail because:</span>
<ul>
<li>You are on the CC list for the bug.</li>
</ul>
</body>
</html>