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

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++.