"To whom" in pied-piped infinitive relative clauses

Question

In English wh raised from, or in situ in, a direct object or prepositional object, you can almost always use "who" at least as well as "whom",¹ and in some cases you can only use "who":

• Who/whom did you meet?
• Who/whom were you talking to?
• To who/whom were you talking?
• I asked her who/whom to talk to.
• I asked her who/?whom she met.
• I asked her who/whom she was talking to.
• What did you give to who/whom?
• You gave it to who/*whom?!
• I already told you about what to talk about to who/whom.
• I already told you about what to talk about, to who/whom.
• I already told you about what to talk about, and to who/whom.

But there seems to be at least one case where whom is actually required: when pied-piping a to-adjunct out of a relative clause, or at least an infinitive one:

• I already told you about the man to *who/whom to talk.

Is there any principled reason for this one construction to be different from all the others?

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More specifically, is there any way to account for it in mainstream generative grammar?

As I understand it, all of the examples above are usually explained by the fact that "who" is marked with (abstract) accusative/ACC or oblique (depending on your favorite flavor of Case theory), but modern English allows realizing accusative "who" as either "who" or "whom", or similar. But then why does it not allow it in this one sentence?

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Or, alternatively: In a construction grammar it's easy to account for: the pied-piped infinitive relative clause construction just overrides whatever is inherited from the parent construction(s), so that it requires "whom".

But such an unmotivated idiosyncrasy should eventually die away.² And "whom" has had a long time to do so, and all the other uses of whom are well on their way, but this construction seems to be stubbornly insisting on it without exception, even today. So, why?

My best guess—which seems like a pretty bad guess—is the very minor phonological infelicity of "to who to", which doesn't appear in any of the other examples.

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_{1. At least by actual speakers of English—even in formal writing. Most prescriptive grammar writers claim it's incorrect, but then they also mandate hypercorrecting to "whom" even in instances that are clearly nominative, like the infamous "Whom shall I say is calling?" And of course most of them don't allow wh-raising without pied-piping in the first place, so half of these examples can't even arise for them.}

_{2. Also, of course any CxG framework will leave room for a few nuts that just don't crack right, because languages don't have to be perfect. But that's a last resort fallback, not the default assumption.}

Answer 1

As pointed out by sumelic in the comments, my corpus is woefully incomplete, and I missed the important generalization. The preposition doesn't matter: "I already told you about the man to/with/about *who/whom to talk." The only reason all the examples I found used "to" is that most of the examples of pied-piped who/whom happen to use "to". So, obviously, my wild guess about phonotactics is completely wrong. Most other variables, like which position you extract from, don't matter either. The main thing that matters is pied-piping, but it's not quite that simple.

If you go through the catalog in Ivan Sag's 1997 English Relative Clause Constructions,¹ trying to cram a who/whom into every possible (non-subject) position:

• The constructions (actually construction/site pairs, not separate constructions, at least in Sag's terms) that require focus stress (although Sag doesn't get into that) prefer who, and the in-situ one requires it.
• One other (seemingly arbitrary) construction at least slightly prefers who.
• The constructions that pied-pipe prefer whom, and the infinitival one requires it.
• Everything else accepts either.

This strongly implicates register, as sumelic suggested. "You gave it to WHO?!" is the kind of thing people say in the schoolyard, and "I already told you about the man to *who/whom to talk" is the kind of thing people say in court.

But pragmatics would only explain the generalizations, not the idiosyncrasies. Plus, even if you imagine a lawyer using the in-situ stressed question in court, she still can't say "whom" without it sounding weird. So, the pragmatic register distinction has been grammaticalized—some feature of each construction is involved in the pronoun choice.

This makes perfect sense from a Construction Grammar view of synchronic language (good but not perfect generalization from "syntactic nuts") and a Goldbergian view of language evolution (pragmatics, and the drive to generalize, cause "whom" to die out faster in some constructions than others in a way that's pretty predictable, but not perfectly so). Especially considering Luke Sawczak's point about cliches in comments—e.g., most learners have probably heard the fixed "to whom it may concern" far more than any other phrase in the same construction. I don't know how to fit it into the Chomskyan MP paradigm (even though I think Chomsky originally invented the "grammaticalized pragmatic" idea…), but that's fine.

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_{1. If you're interested in the actual construction hierarchy and the explanation for it, not just the catalog, skimming Peter Culicover's 2013 book Grammar and Complexity, he seems to improve on Sag. I suspect Thomas Hoffman's 2011 book Preposition Placement in English improves it in different ways—including explicitly taking register (and dialect) into account—but I haven't got a copy. On the other hand, Sag is a relatively short paper available as a free preprint, as opposed to a whole book.}