`error: exception specification of 'operator()<Iterator>' uses itself` when dealing with string_view

Question

Consider following code:

run on gcc.godbolt.org

#include <string_view>

struct Iter
{
    Iter() {}
    Iter(std::string_view) {}

    Iter begin() const
    {
        return *this;
    }
};

It's a stripped down iterator, which is supposed to be used as for (auto x : Iter(...)) (exactly the same as fs::directory_iterator). I've removed end() and all the overloaded operators for brevity, since they don't affect the error.

This code compiles with -std=c++20 (with both GCC and Clang), but once I switch to -std=c++2b, GCC and Clang with libstdc++ reject it, while Clang with libc++ still accepts it.

Here's the gist of the error:

iterator_concepts.h:945:6: error: exception specification of 'operator()<Iter>' uses itself
...
string_view:153:7: note: while checking the satisfaction of concept 'contiguous_range<const Iter &>' requested here
...
string_view:153:15: note: while substituting template arguments into constraint expression here
          && ranges::contiguous_range<_Range>
                     ^~~~~~~~~~~~~~~~~~~~~~~~
<source>:10:16: note: while checking constraint satisfaction for template 'basic_string_view<const Iter &, Iter>' required here
        return *this;
               ^
<source>:10:16: note: in instantiation of function template specialization 'std::basic_string_view<char>::basic_string_view<const Iter &, Iter>' requested here

The error doesn't happen if the function isn't named begin. Apparently return *this; considers the string_view constructor, which checks some concepts for Iter, which includes checking the begin itself.

What is going on and how do I fix this error?

---

Reported a bug for libstdc++.

Answer 1

This code compiles with -std=c++20 (with both GCC and Clang), but once I switch to -std=c++2b, GCC and Clang with libstdc++ reject it, while Clang with libc++ still accepts it.

Clang 14 with libstdc++ fails to compile it because Clang 14 had a buggy implementation of concepts and cannot compile GCC's <ranges> header. Clang 15 with a recent version of libstdc++ compiles it OK.

GCC 12.1 failed to compile due to a cycle in constraint checks for one of string_view's constructors. GCC 12.2 compiles OK, because it includes the change from P2499R0 "string_view range constructor should be explicit".

tl;dr C++23 was still under development in January 2022 and support was patchy.