' All SVC, with subject complements of a noun, a gerund and an infinitive, respectively. PS: On re-reading your post, Hanuman, I now see that you may be confusing the form of the complement with its designation , which indicates the sentence part to which it refers. 'To sing' is indeed a verb form, but it is in this sentence called a subject complement because it refers to the subject 'desire', not the verb 'is'.
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