"Whose ever" instead of "Who's ever"

Question

I sometimes hear non-natives say "Whose ever" instead of "Who's ever". Then maybe they confuse it with "Whoever's"?

Where could this come from and is it any way correct?

• You can take whose ever/who's ever/whoever's seat you want.

Answer 1

Two of the three options are correct:

• You can take whose ever seat you want. (More formal)
• You can take whoever's seat you want. (More colloquial)

"Who's ever" is incorrect (unless it stands for "who is ever" or "who has ever", which it can't in this sentence).

The possessive form of "who" is always "whose", although "who's" is a commonly seen misspelling.

(Fowler's Modern English Usage, 1965, has: whoever. Forms. Subjective: whoever, whosoever (emphatic), who-e'er (poetic), whoso (archaic), whosoe'er (poetic). Objective: whomever (rare), whoever (colloq.), whomsoever (literary), whomsoe'er (poetic), whomso (archaic). Possessive: whose ever, whoever's (colloq.), whosesoever (archaic).)

Note: I was unaware until I read Rompey's answer that there exists an alternative one-word spelling ("whosever"); this isn't mentioned in the 1965 edition of Fowler's but is included in the 1996 edition.

Answer 2

Whatever reliable dictionary you look up "whosever" in, you'll find out that it is a single word meaning "belonging to whichever person/of whomever".

For example:

The choice, whosever it was, is interesting.

So, no apostrophe is needed in

You can take whosever seat you want.