The NA28 text of Luke 6.20b reads: Μακάριοι οἱ πτωχοί, ὅτι ὑμετέρα ἐστὶν ἡ βασιλεία τοῦ θεοῦ. (A minority of witnesses, especially among Syriac, and Coptic versions and Marcion [acc. to Tertullian], harmonize Luke to Matthew's third-person beatitudes; the Matthean beatitudes are not subject to harmonization to Luke in this regard.)
Here is Watson's challenge, which I quote at length (see Gospel Writing, pp. 160–61n. 6):
Luke's μακάριοι οἱ πτωχοί ὅτι ὑμετέρα . . . should not be translated, "Blessed are you poor, for yours . . .," as in EVV (my thanks to Mark Goodacre for alerting me to this point). The awkward shift from ostensibly third-person to second-person discourse is a sign of secondariness vis-à-vis Matthew, as in the fact that ὑμετέρος occurs only in Luke among the synoptists (Lk. 6.20; 16.12; cf. Acts 27.34). Cf. GTh 54 as correctly translated by T. O. Lambdin: "Jesus said, 'Blessed are the poor, for yours is the kingdom of heaven'" (NHL, p. 124).The predicate construction (adjective-article-noun; lit. "blessed the poor ones") does seem to suggest a third-person sense, and the translation "blessed are you who are poor" seems to be suggested by the second person possessive pronoun, "yours" (ὑμετέροι), in the second half of the beatitude. Since there is no verb—whether "you are" (ἐστέ) or "they are" (εἰσίν)—we have to supply a verb in order to complete the sense implied by the Greek construction; the syntax of adjective, article, and noun (including their case) is determinative. As I wondered why I hadn't recognized Luke's shift from the third- to second-person, I realized that I had taken "blessed the poor" (μακάριοι οἱ πτωχοί) as a vocative construction and so rendered the words as direct address: "Hey; you who are poor. You are blessed." But the article, "blessed the poor" (μακάριοι οἱ πτωχοί), does suggest a nominative case, which I think would support a third-person interpretation.
All very good. As a final step, I consulted my ragged copy of Blass, Debrunner, and Funk's A Greek Grammar of the New Testament and Other Early Christian Literature, fully expecting the authorities that be (viz., BDF) to confirm this observation. Alas, they did not (see BDF §147, p. 81):
Even where the nominative is still formally distinguished from the vocative, there is still a tendency for the nominative to usurp the place of the vocative (a tendency observable already in Homer). In the NT this is the case (1) generally with adjectives used alone or without a substantive where the vocative is clear; (2) with additions of all kinds to the vocative (Attic σὺ ὁ πρεσβύτερος, Πρόξενε καὶ οἱ ἄλλοι), especially with participles (§412(5)) which hardly ever form the vocative. (3) Attic used the nominative (with article) with simple substantives only in addressing inferiors, who were, so to speak, thereby addressed in the 3rd person (Aristoph., Ra. 521 ὁ παῖς, ἀκολούθει). The NT (in passages translated from a Semitic language) and the LXX do not conform to these limitations, but can even say ὁ θεός, ὁ πατήρ, etc., in which the arthrous Semitic vocative is being reproduced by the Greek nominative with article.In the list of examples of (2) ("with additions of all kinds to the vocative, especially with participles which hardly ever form the vocative"), BDF offer Luke 6.25: οὐαὶ ὑμῖν, οἱ ἐμπεπλησμένοι ("Woe to you, you who are satisfied"), which of course belongs to the same context as the Lukan beatitudes. It hardly matters that the first beatitude has an adjective-article-noun construction, while the second and third woes (and also the second and third beatitudes!) have participles. The point is that the entire context—from the possessive pronoun ὑμετέροι, the personal pronoun ὑμῖν in the second and third woes, the verbs in the second and third beatitudes and in the first three woes—support the reading that μακάριοι οἱ πτωχοί in Luke 6.20b is a vocative phrase and not nominative.
Therefore, Watson (and Goodacre) are wrong to insist on translating the Lukan beatitudes, "Blessed are those who are poor, for yours is the kingdom of God." More importantly, perhaps, they are wrong to see in Luke's beatitudes any awkward shift from a Matthean third-person construction to a now-redacted Lukan second-person construction. The entire beatitude is a second-person construction: "Blessed are you who are poor, for yours is the kingdom of God." Even if Luke is interpreting Matthew (a position I am not interested to refute here), the first Lukan beatitude does not preserve evidence of Luke's redactional work on Matthew's blessings.