The issue sometimes does come up on writer forums: whoever vs whomever.
For example, a type of sentence that can cause problems is one such as this:
Tom was willing to date whoever/whomever was waiting for him on the other side of the door.The problem in that sentence is that the pronoun (relative word) seems to have to fulfill two opposing syntactic functions at the same time. It wants to fulfill the case requirement of the matrix clause (pronoun be in the accusative case) and the case requirement of the relative clause (pronoun be in the nominative case). In this specific example, the nominative case ("whoever") sounds better to my ear, while the accusative case ("whomever") seems to sound a bit stilted.
So, let's see if there's some stuff on this in a reference grammar.
- Tom was willing to date whoever was waiting for him on the other side of the door. (my preference)
- Tom was willing to date whomever was waiting for him on the other side of the door.
In the reference grammar by Huddleston and Pullum et al., The Cambridge Grammar of the English Language, first published in 2002, on pages 1073-4:
It is example [18.iv] and its discussion that is very applicable here. This excerpt from CGEL seems to be succinct and informative.. . . In constructions with personal who and whoever, the pronoun has to satisfy the case requirements of both the relative clause and the matrix clause in which the whole NP is functioning. Compare:
i. [Whoever is responsible for the damage] must pay for it.
ii. He will criticise [whomever she brings home].
iii. ?[Whomever he marries] will have to be very tolerant.
iv. ?She lunches with [whomever is going her way after morning classes].
In [i] both the whole NP (bracketed) and the relativised element (underlined) are subject of their respective clauses: the nominative form matches both requirements. In [ii] both the whole NP and the relativised element are objects, and the accusative is fully acceptable though somewhat formal in style. In [iii-iv], however, there is a clash between the function of the whole NP and that of the relativised element -- respectively subject and object in [iii], object of a preposition and subject in [iv] -- and the result is at best very questionable. Whoever would be preferable in both, but many would regard it as less than fully acceptable in formal style.
I'd be surprised to find this information in a grammar usage manual, or style manual; and there's a good chance that it might not be in many reference grammars (I don't think Quirk et al. 1985 has it -- edited-to-add: they do have something on page 1052 as part of note 'e', but their way of looking at this issue seems to be different). There might be a chance that some blog by a professional grammarian might have this info--maybe. But I don't think there's much of a chance that an online grammar source will.