https://gcc.gnu.org/bugzilla/show_bug.cgi?id=120305
--- Comment #10 from Jonathan Wakely <redi at gcc dot gnu.org> --- (In reply to Jonathan Wakely from comment #7) > An iota_view with unsigned long elements (i.e. 64-bit integers) must use > something wider than 64 bits for the difference_type of its iterators. The > C++20 standard allows that to be an integer type or an integer-like class > type. > > In strict standard-conforming mode, we do not use the non-standard __int128 > type for iota_view iterators, so we use an integer-like class type. Sorry, I slightly misremembered the details and this is not quite correct. We use __int128 for the difference_type even in strict mode. The problem is that the Cpp17InputIterator requirements require the iterator's difference_type to be a standard integer type, and in struct modes __int128 is not considered a standard integer type (it's a non-standard extension). In non-strict -std=gnu++20 mode we consider __int128 to be a standard integer type, and so iota iterators meet the Cpp17InputIterator requirements. So the end result is the same, the difference is just in the precise details of *why* iota view iterators do not meet the Cpp17InputIterator requirements.
