Proposed resolution:

Change [locale.codecvt.virtuals], p7:

Effects: Places characters starting at to that should be appended to terminate a sequence when the current stateT is given by state.²³⁷⁾ Stores no more than (to_limit -to) destination elements, and leaves the to_next pointer pointing one beyond the last element successfully stored. codecvt<char, char, mbstate_t> stores no characters.

[locale.codecvt.virtuals], para 7 says (regarding do_unshift):

Effects: Places characters starting at to that should be appended to terminate a sequence when the current stateT is given by state.²³⁷⁾ Stores no more than (to_limit - to) destination elements, and leaves the to_next pointer pointing one beyond the last element successfully stored. codecvt<char, char, mbstate_t> stores no characters.

The following objection has been raised:

Since the C++ Standard permits a nontrivial conversion for the required instantiations of codecvt, it is overly restrictive to say that do_unshift must store no characters and return noconv.

[Plum ref _222152Y50]