Proposed resolution:

Change [locale.codecvt.virtuals], p8:

Returns: An enumeration value, as summarized in Table 76:

...

codecvt<char,char,mbstate_t>, returns noconv.

[locale.codecvt.virtuals], para 8 says:

codecvt<char,char,mbstate_t>, returns noconv.

The following objection has been raised:

Despite what the C++ Standard says, unshift can't always return noconv for the default facets, since they can be nontrivial. At least one implementation does whatever the C functions do.

[Plum ref _222152Y62]