I came across these lines in a hymn:
Cherubim and seraphim falling down before Thee,
Which wert and art, and ever more shalt be.
I noticed that "wert", "art", and "shalt" were used with the subject "which" in the last line instead of which "thou." At first I thought this was just a grammatical mistake on the side of the hymn writer, but then I kept seeing such things where verbs in second person singular form are used with indefinite pronouns such as "which" or "who". Another example is give in this StackExchange question whose answer doesn't really answer my question.
So now I'm wondering, is it correct to use second person singular verbs with indefinite pronouns if the indefinite pronoun refers to a second person singular pronoun (in the hymn, "which" reffers to "Thee" from the last line)?