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Because std::span<Incomplete> can be useful, it makes sense to define std::span<Incomplete>::iterator such that Incomplete is not an associated class, and so the compiler won't attempt to complete it when doing ADL for span iterators (including during the definition of std::span<Incomplete>::const_iterator which checks that iterator satisfies std::input_or_output_iterator). We can't make this change for std::vector<Incomplete> because it would change the mangled name of std::vector<Incomplete>::iterator which would affect the mangled names of templates and functions written by users. We can do the same thing for std::basic_stacktrace<Alloc> just so that Alloc is not an associated class. This is probably less beneficial, as Alloc can't be incomplete, and using SomeAllocator<Incomplete> as the allocator parameter doesn't seem useful. But simply making the stacktrace iterator not use Alloc for ADL lookup seems worthwhile. This is doable because std::stacktrace is part of C++23 so its ABI isn't considered stable yet. libstdc++-v3/ChangeLog: * include/std/span (span::__iter_tag): Declare nested type. (span::iterator): Use __iter_tag as second template argument. * include/std/stacktrace (basic_stacktrace::iterator): Use _Impl as second template argument.