#Implementation

 - `operator==(SplitStringIterator const&)` only compares pointers for equality, not lengths. This might not be intended.
 - The `ForwardIterator` concept isn't actually fulfilled: For that, `operator*` has to return a `reference`! (And it makes sense, too: You cannot provide a multipass guarantee in case the contents get modified, i.e. a previously different `CharT` value gets changed to `delim`.)
 - Why restrict `delim` to `CharT`, when it could just as easily be a `std::basic_string_view<CharT>`? The `advance_()` logic would have to be updated for the varying length, but that's the only non-type change I can see that would be necessary. (Though one could add a conversion or a special case for single `CharT` delimiters.)

> **Small caveat**: If `parts_indexable` had a longer lifetime than `data` (e.g. it got returned by `foo`), all its contents would dangle (though this is inherent problem of `std::string_view`, and not of your implementation). This could be fixed by making explicit copies by changing the type of `parts_indexable` to `std::vector<std::string>`. (Not a criticism of your work, just a note for users of this iterator).