You mean a subformula? This was my initial attempt, before relying on taps, but it doesn't work. Here's why (two issues that I opened here a few weeks ago):

https://github.com/Homebrew/homebrew/issues/24060?source=c
and
https://github.com/Homebrew/homebrew/issues/24063

In short, Homebrew subformulae are not independent, as build environments, from the formula they are contained within. This architectural constraint breaks compilation of Guile 1.8 in this case.