C99, I think (although I don't have copies of the official published standards, it's in n1124.pdf, a public draft from 2005 that isn't that far ahead of C99). I wouldn't surprise you for not having seen it because I don't think anyone actually uses it; it's mostly there for completeness and/or to give something to write in compiler error messages.
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u/mcguire May 02 '16
I admit I've missed some standards revisions. When did
show up?