relative clauses - What part of speech is "the closest" in this case?

What is the closest you've ever come to losing your life?

（From : https://www.quora.com/What-is-the-closest-youve-ever-come-to-losing-your-life）

Apparently, it comes from the construction "come close to doing" , so "the closest" should be an adverb (in superlative degree) . But relative clauses cannot modify an adverb.（I follow the CGEL by Quirk et al.）

Someone would view it as a NP where the noun has been omitted(the closest situation). But this analysis is also flawed, because a NP cannot fit in the relative clause that follows.

Is there a better idea ？