Bryan Garner is probably the foremost authority on American English usage and certainly the foremost authority on legal usage. Everyone who cares about language should have his magnificent Dictionary of Modern American Usage, and every lawyer and law student should also own his Dictionary of Legal Usage.

But, yesterday, I think Garner made an error. Over at LawProse, he has a typically informative and precise post on “whoever” vs. “whomever.” Here, though, is the last paragraph:

It’s worth it to master using these terms. Whoever you’re writing for will appreciate the effort, and your grammatical precision will reflect well on you. (The preceding sentence is a tricky one: Whoever is the subject of will appreciate.)

I think Garner has this wrong. “Whoever” is not the subject of “will appreciate”; the entire clause “Whoever you’re writing for” is the subject of “will appreciate.” The important thing for these purposes is the function of “who(m)ever” within that clause: “(Who(m)ever you’re writing for) will appreciate the effort….” In that clause, “you” is the subject, and “who(m)ever” is the object of the preposition “for.” “You’re writing for whom? You’re writing for him.”

Here is GrammarBook, stating the rule:

When the entire whoever/whomever clause is the subject of the verb that follows the clause, look inside the clause to determine whether to use whoever or whomever.

Examples:
Whoever is elected will serve a four-year term.
Whoever is the subject of is elected. The clause whoever is elected is the subject of will serve.

Whomever you elect will serve a four-year term.
Whomever is the object of elect. Whomever you elect is the subject of will serve.

Therefore, in Garner’s sentence, the correct choice is “whomever”:

Whomever you’re writing for will appreciate the effort, and your grammatical precision will reflect well on you.

I post this with some trepidation, as I have never known Bryan Garner to make an error of this (or any) sort.  So, who’s right?

UPDATE (Sept 1):   I am pleased to report that Bryan Garner has changed his post, replacing “you’re writing for” with “your readers are.” The last paragraph now reads as follows:

It’s worth it to master using these terms. Whoever your readers are will appreciate the effort, and your grammatical precision will reflect well on you. (The preceding sentence is a tricky one: Whoever is the subject of will appreciate.)

Now, “Whoever” is indeed correct. Unfortunately, though, the parenthetical explanation is still wrong. Just as before, “Whoever” is not the subject of “will appreciate”; the entire clause “Whoever your readers are” is the subject of “will appreciate.” And, just as before, the important thing for these purposes is the function of “Whoever” within that clause: “(Whoever your readers are) will appreciate the effort….” The word “are,” a form of the verb “to be,” takes the subject complement “Whoever.”