Resolving in Properties currently says (italics mine):

If a property value contains one or more arbitrary substitution functions, and those functions are themselves syntactically valid, the entire value’s grammar must be assumed to be valid at parse time.

My understanding is that now, we don't do any syntax checking on ASFs until computed-value time, and the clause about them being syntactically valid should be removed from here. I think this just got missed in the recent ASF rewrite.

cc: @tabatkins